A
Acceptable Use Policy (AUP)
A set of rules defined by an email service provider that specifies what types of email are permitted and prohibited on the platform. AUPs protect shared sending infrastructure by preventing abuse that could damage deliverability for all users. Violations can result in account suspension. See Acceptable Use Policy for Email.Aggregate Report (RUA)
A daily XML report sent by receiving mail servers to the address specified in your DMARC record. Aggregate reports summarize authentication results (SPF, DKIM, DMARC pass/fail) for all emails received from your domain, helping you identify unauthorized senders and misconfigured services. See Understanding SPF, DKIM, and DMARC.Alignment
In DMARC, alignment means the domain in the visible “From” header matches the domain that passed SPF or DKIM authentication. Without alignment, an email can pass SPF or DKIM but still fail DMARC. There are two modes: strict (exact domain match) and relaxed (organizational domain match, allowing subdomains). See Understanding SPF, DKIM, and DMARC.AMP for Email
An email content type (text/x-amp-html) that allows interactive elements like carousels, forms, and live content within the email itself. Support is limited to Gmail, Yahoo Mail, and Mail.ru. AMP emails require sender registration with each supporting provider.
See Email Content Types.
API Error Code
A machine-readable identifier returned in Lettr API error responses (alongside human-readable error messages) to indicate the specific type of failure. Common error codes includevalidation_error, invalid_domain, unconfigured_domain, send_error, template_not_found, and resource_already_exists. Error codes enable programmatic error handling and logging.
See API Reference Introduction.
API Key
A secret token used to authenticate requests to the Lettr API. API keys should be stored in environment variables and never committed to source code or shared publicly. Each key can be scoped to specific permissions. See API Keys and Security Best Practices.ARC (Authenticated Received Chain)
An email authentication protocol (RFC 8617) that preserves authentication results when email is forwarded or modified by intermediaries (mailing lists, forwarding services). ARC creates a chain of cryptographically signed headers showing each server’s authentication verification, allowing final recipients to trust authentication results despite email modifications that would normally break DKIM signatures. See Understanding SPF, DKIM, and DMARC.ARF (Abuse Reporting Format)
A standardized email format (defined in RFC 5965) used by mailbox providers to report spam complaints back to senders through feedback loops. ARF reports include the original message headers and identify the complaint type, enabling ESPs to process complaints and suppress complaining recipients automatically. See What Are Feedback Loops?.Authentication-Results Header
An email header added by the receiving mail server that shows the pass/fail status of SPF, DKIM, and DMARC checks. This header is the primary tool for diagnosing authentication issues. See Email Headers Explained.Auto-Submitted Header
An email header (Auto-Submitted: auto-reply or auto-generated) that indicates a message was generated automatically rather than by a human. Auto-submitted headers identify out-of-office replies, bounce notifications, mailing list responses, and automated system messages. Properly setting this header prevents auto-reply loops and helps receiving systems classify automated messages.
See Reply Tracking and Email Headers Explained.
B
Backscatter
Bounce messages sent to an innocent third party whose email address was forged as the sender of spam. Proper SPF, DKIM, and DMARC configuration prevents your domain from being used in backscatter attacks.Base64 Encoding
A MIME encoding method that converts binary data into ASCII text using a 64-character alphabet (A-Z, a-z, 0-9, +, /). Base64 is used for encoding email attachments (images, PDFs, documents) and HTML email content in some clients. While less efficient than binary (33% size overhead), Base64 ensures safe transmission through systems that only support 7-bit ASCII. See Attachments.Batch Sending
Sending a single API request with multiple recipients rather than making individual requests per recipient. Lettr supports up to 50 recipients per batch request. Batch sending reduces API calls and improves throughput for high-volume senders. See Batch Sending.BIMI (Brand Indicators for Message Identification)
An email standard that allows senders to display their brand logo next to messages in recipient inboxes. BIMI requires a fully enforced DMARC policy (p=quarantine or p=reject) and, for most mailbox providers, a Verified Mark Certificate (VMC). The BIMI record is published as a DNS TXT record at default._bimi.yourdomain.com.
See BIMI: Brand Indicators for Message Identification.
Blocklist (Blacklist)
A database of IP addresses or domains identified as sources of spam. Mailbox providers and spam filters check incoming email against blocklists. Being listed on a major blocklist (Spamhaus, Barracuda, SORBS) can severely impact deliverability. Monitor your status and request delisting if listed. See Sending Reputation.Bounce
An email that cannot be delivered to the recipient. Bounces are classified as hard (permanent failure, e.g., address doesn’t exist) or soft (temporary failure, e.g., mailbox full). Hard bounces should result in immediate suppression. Soft bounces are retried automatically. See Bounce Codes Reference and Bounce Diagnosis.Bounce Rate
The percentage of sent emails that bounce. A healthy bounce rate is below 2%. Rates above 5% indicate a list quality problem and can damage sender reputation. Calculate as:(bounced emails / sent emails) x 100.
See Bounce Diagnosis.
Bulk Sender
In the context of Google and Yahoo’s 2024 requirements, any sender who sends 5,000 or more emails per day to Gmail or Yahoo recipients. Bulk senders must implement SPF, DKIM, DMARC, one-click unsubscribe, and maintain a spam complaint rate below 0.3%. See Google & Yahoo Sender Requirements.C
CAN-SPAM Act
A US federal law (2003) governing commercial email. Requires accurate header information, non-deceptive subject lines, identification as an advertisement, a physical mailing address, a working unsubscribe mechanism (honored within 10 business days), and responsible monitoring of third-party senders. Penalties reach $50,120 per non-compliant email. See CAN-SPAM Requirements.Character Encoding
The method of representing text characters as bytes in email content. UTF-8 is the modern standard that supports all Unicode characters (emoji, non-Latin scripts, symbols). Older emails may use ISO-8859-1 (Latin-1), Windows-1252, or other legacy encodings. The character encoding is declared in theContent-Type header (e.g., text/html; charset=utf-8). Mismatched encoding declarations and content cause rendering issues.
See Email Parsing and Rendering Issues.
CASL (Canadian Anti-Spam Legislation)
A Canadian federal law (2014) regulating commercial electronic messages sent to or from Canadian computer systems. CASL is stricter than CAN-SPAM — it requires prior consent (either express or implied) before sending, rather than allowing an opt-out model. CASL applies regardless of the sender’s location if the recipient is in Canada. Penalties can reach CAD $10 million per violation for organizations. See CASL (Canadian Anti-Spam Law).Click Tracking
A technique that detects when a recipient clicks a link in an email by routing links through a tracking server that records the click before redirecting to the destination URL. Click tracking provides more reliable engagement data than open tracking because it requires deliberate user action and is not affected by image-blocking or privacy proxies. See Tracking Accuracy.CID (Content-ID)
A standard for referencing inline images and attachments within multipart email messages. CID URIs (e.g.,<img src="cid:logo123">) link to attachments by their Content-ID header value rather than external URLs. This keeps images embedded within the email rather than loaded from remote servers, improving rendering reliability and avoiding external resource blocking.
See Attachments.
CNAME Record
A DNS record type that creates an alias from one domain name to another. Lettr uses CNAME records for sending domain verification and tracking domain configuration. A CNAME points your subdomain to Lettr’s infrastructure, delegating email authentication.Complaint
When a recipient marks an email as spam or junk in their email client. Complaints are reported back to senders through feedback loops. A complaint rate above 0.1% is a warning sign; above 0.3% risks deliverability penalties from major providers. See List Hygiene.Content Block
In the Topol email editor, a content block is a discrete element added to a structure container — such as text, image, button, divider, spacer, HTML, or video. Content blocks are the building blocks of email templates. Each block type has configurable properties (alignment, padding, colors, fonts) and can include merge tags for dynamic content. See Topol Email Editor and Saved Blocks.Content Filtering
The process by which receiving mail servers analyze email content (subject lines, body text, HTML structure, links, images) to determine whether a message is spam. Filters use machine learning, keyword analysis, URL reputation, and sender history. See Emails Landing in Spam.Custom Return-Path
A configuration that replaces the ESP’s default bounce domain with a subdomain of your own domain as the envelope from address. This achieves SPF alignment alongside DKIM alignment, providing dual-factor DMARC compliance and removing the “via” label in Gmail. See Custom Return-Path and MAIL FROM.D
Dark Mode (Email)
A display setting in email clients that inverts or adjusts colors to show light text on dark backgrounds. Email clients handle dark mode differently — some apply full color inversion, some partial inversion, and some leave emails unchanged. Defensive CSS techniques (explicit colors, avoiding pure black/white, transparent image backgrounds) ensure readability across all dark mode behaviors. See Dark Mode Email Design.Data Minimization
A principle under GDPR that requires collecting and retaining only the personal data necessary for the stated purpose. For email senders, this means avoiding storing unnecessary recipient data in email metadata and defining clear retention periods for email logs and events. See Data Retention and Deletion.Data Processing Agreement (DPA)
A legally required contract between a data controller (your organization) and a data processor (such as an ESP) under GDPR. A DPA specifies how personal data will be processed, what security measures are in place, and the obligations of each party regarding data protection. See GDPR and Email Sending.Dedicated IP
An IP address assigned exclusively to a single sender. Dedicated IPs give you full control over your sending reputation, since no other sender’s behavior affects your IP. They require a warm-up period and consistent sending volume to maintain reputation. Best suited for senders with 100,000+ emails per month. See Dedicated vs Shared IPs.Deferral
A temporary delay in email delivery caused by the receiving server responding with a 4xx status code, indicating it is not ready to accept the message right now. Deferrals are common during warm-up and when sending to providers with strict rate limits. The sending server retries automatically. See How Email Delivery Works and Bounce Codes Reference.Delay
A temporary delay in email delivery caused by the recipient server’s deferral response (4xx SMTP codes) or rate limiting. Delay events indicate the email is queued for retry and will be delivered later if subsequent attempts succeed. Delays are normal during IP warm-up, high-volume sending, or when recipient servers are experiencing load. Distinct from a soft bounce (which may convert to hard bounce) or delivery (which indicates success). See Event Types and Bounce Codes Reference.Deliverability
The ability of an email to reach the recipient’s inbox (as opposed to the spam folder, or being rejected entirely). Deliverability is influenced by sender reputation, authentication, content quality, list hygiene, and engagement metrics. Distinct from delivery, which simply means the receiving server accepted the message. See What Is Email Deliverability? and Deliverability Best Practices.Delivery
The act of a receiving mail server accepting an email. A “delivered” email has been accepted by the recipient’s server, but may still be placed in spam, a promotions tab, or another folder. Delivery does not guarantee inbox placement. See How Email Delivery Works.DKIM (DomainKeys Identified Mail)
An email authentication protocol that adds a cryptographic signature to outgoing messages. The receiving server uses a public key published in DNS to verify that the email was authorized by the domain owner and wasn’t modified in transit. Unlike SPF, DKIM survives email forwarding. See Understanding SPF, DKIM, and DMARC.DKIM Selector
A unique identifier that specifies which DKIM public key to use for signature verification. The selector is included in the DKIM signature header (s= tag) and is used to look up the public key at selector._domainkey.yourdomain.com. Multiple selectors allow key rotation and multiple signing configurations per domain.
See Understanding SPF, DKIM, and DMARC.
DMARC (Domain-based Message Authentication, Reporting & Conformance)
A policy framework built on SPF and DKIM that tells receiving servers what to do when authentication fails. DMARC adds alignment checks and provides a reporting mechanism. Policies range fromp=none (monitor only) to p=reject (block unauthenticated email).
See Understanding SPF, DKIM, and DMARC.
DNS (Domain Name System)
The system that translates domain names into IP addresses and stores records used for email routing and authentication. Email relies on DNS for MX records (mail routing), TXT records (SPF, DKIM, DMARC), and CNAME records (domain delegation).DNS Propagation
The time it takes for DNS record changes to spread across DNS servers worldwide. Propagation typically completes within 1–2 hours but can take up to 48 hours. During propagation, some servers may see the old records while others see the new ones. See Domain Verification.Domain Connect
An open protocol that enables one-click DNS record configuration through cryptographically signed URLs. When a sender’s domain is hosted on a supporting DNS provider (such as Cloudflare), Lettr can generate a Domain Connect link that automatically adds the required SPF, DKIM, DMARC, and verification records without manual DNS editing. This streamlines domain verification and reduces configuration errors. See Domains Introduction.Domain Reputation
A score or classification that mailbox providers assign to your sending domain based on the quality and history of email sent from it. Domain reputation is independent of IP reputation and is increasingly the primary factor in filtering decisions at providers like Gmail. Factors include bounce rates, complaint rates, spam trap hits, and engagement. See Sending Reputation and IP and Domain Warm-Up Guide.Domain Status
The current state of a sending domain in Lettr’s approval workflow. Domains progress through three statuses: Pending (awaiting manual review), Approved (cleared for sending), and Blocked (prohibited from sending due to policy violations or abuse). Only approved domains can send email. Domain status is returned in API responses and displayed in the Lettr dashboard. See Domains Introduction.DSN (Delivery Status Notification)
A standardized bounce message format (defined in RFC 3464) that reports delivery failures back to the sender. DSNs include machine-readable sections with SMTP status codes, enhanced status codes, recipient addresses, and diagnostic messages. Modern bounce handling relies on parsing DSN fields rather than analyzing freeform bounce text. See Bounce Codes Reference and Bounce Diagnosis.Double Opt-In
A signup process where a new subscriber must confirm their email address by clicking a link in a confirmation email before being added to a mailing list. Double opt-in ensures valid addresses, reduces bounces, and provides documented proof of consent for GDPR compliance. See Email Consent Best Practices and GDPR and Email Sending.E
Email Clipping
When an email client truncates a message that exceeds a size threshold, hiding the remaining content behind a “View entire message” link. Gmail clips emails larger than 102 KB. Clipped content is not rendered, which can hide tracking pixels, unsubscribe links, and important information. See Email Content Rendering Issues and Email Rendering Across Clients.Editor Settings
Customization options in the Topol email editor that control the editing environment, including custom fonts, color palettes, default styles, merge tag definitions, and editor UI preferences. Editor settings can be configured per-team or per-project to standardize design systems and branding across email templates. See Editor Settings and Email Editor Best Practices.EHLO / HELO
SMTP commands used to initiate a connection between sending and receiving mail servers. HELO (defined in the original SMTP spec) identifies the sending server by hostname. EHLO (Extended HELO) is used for ESMTP and requests a list of supported extensions (STARTTLS, AUTH, SIZE, etc.). Modern servers use EHLO; if the receiving server doesn’t support it, the sender falls back to HELO. See SMTP Protocol Basics.Engagement
How recipients interact with your emails — opens, clicks, replies, forwards, and time spent reading. Mailbox providers use engagement signals to determine inbox placement. High engagement improves deliverability; low engagement signals that recipients don’t want your email.Enhanced Status Code
A three-part code (e.g.,5.1.1) included in SMTP bounce responses that provides more detail than the basic three-digit reply code. The format is class.subject.detail, where the class indicates success (2), temporary failure (4), or permanent failure (5).
See Bounce Codes Reference.
Envelope From (Return-Path)
The email address used during the SMTP conversation to indicate where bounce messages should be sent. This is different from the “From” header that recipients see. SPF checks are performed against the envelope from domain. Also called the bounce address or MAIL FROM. See Email Headers Explained and Custom Return-Path and MAIL FROM.ESMTP (Extended SMTP)
An extension of the SMTP protocol that adds support for features like authentication, TLS encryption, and 8-bit content. ESMTP is negotiated when the sending server uses theEHLO command instead of the older HELO. Virtually all modern mail servers use ESMTP.
See SMTP Protocol Basics.
ESP (Email Service Provider)
A company that provides infrastructure and tools for sending email at scale. ESPs handle server management, IP reputation, authentication, deliverability optimization, and compliance. Lettr is an ESP focused on transactional email. See What Is an Email Service Provider?.Exchange Online Protection (EOP)
Microsoft’s email filtering service for Outlook.com, Hotmail, and Microsoft 365. EOP performs connection filtering, sender reputation checks, and SPF/DKIM/DMARC validation as the first layer of Microsoft’s spam defense, before content-level filtering by SmartScreen. See Outlook / Microsoft 365 Delivery Issues.Exponential Backoff
A retry strategy where the wait time between retry attempts increases exponentially (e.g., 1 minute, 5 minutes, 30 minutes, 2 hours). Used in webhook delivery and SMTP retry logic to avoid overwhelming a server that is temporarily unable to accept requests. See Webhook Delivery Failures.Express Consent
Under CASL and GDPR, an affirmative opt-in where the recipient explicitly agrees to receive messages. Express consent does not expire under CASL (valid until withdrawn). Obtaining it requires clearly disclosing who is requesting consent, why, and how to withdraw it. See CASL (Canadian Anti-Spam Law) and Email Consent Best Practices.F
Feedback Loop (FBL)
A service provided by mailbox providers (Gmail, Outlook, Yahoo) that notifies senders when recipients mark their email as spam. Senders register for FBLs to receive complaint notifications and are expected to suppress complaining recipients immediately. Lettr processes FBL reports automatically and adds complainers to your suppression list. See What Are Feedback Loops?.Focused Inbox
Outlook’s email sorting feature that separates incoming messages into Focused (important) and Other (lower priority) tabs. Unlike Gmail’s multi-tab system, Focused Inbox uses a two-tab model. Messages in the Other tab are still delivered but receive less visibility. See Outlook / Microsoft 365 Delivery Issues.Forensic Report (RUF)
A DMARC report that provides detailed information about individual emails that failed authentication. Unlike aggregate reports (which are summaries), forensic reports include message headers or full message content. Many mailbox providers do not send forensic reports due to privacy concerns.From Header
The email address displayed to the recipient as the sender. This is the address that DMARC alignment checks against. It can differ from the envelope from, which is used for SMTP delivery and bounce handling.G
GDPR (General Data Protection Regulation)
An EU regulation governing the processing of personal data, including email addresses. For email senders, GDPR requires a lawful basis for processing (consent, legitimate interest, or contractual necessity), grants data subjects rights (access, erasure, portability), and mandates privacy notices and data protection measures. See GDPR and Email Sending.Generation Failure
A webhook event indicating a template failed to render before transmission. Generation failures occur when template syntax errors, missing merge tags, or invalid substitution data prevent rendering. These errors occur before the email enters the SMTP transmission phase and should be caught during development and testing. See Webhook Event Types and Template Language.Generation Rejection
A webhook event indicating template content was rejected during the generation phase before transmission. Generation rejections occur when content fails validation rules (prohibited content, missing required fields). These errors occur before the email enters the SMTP transmission phase. See Webhook Event Types.Google Postmaster Tools
A free service from Google that provides Gmail-specific data about your sending domain, including domain reputation, spam rate, authentication pass rates, and delivery errors. Google Postmaster Tools is the most important diagnostic tool for troubleshooting Gmail deliverability, as this data is not available from any other source. See Gmail-Specific Delivery Issues.Graymail
Email that isn’t spam but isn’t wanted either — newsletters the recipient signed up for but no longer reads, promotional emails from legitimate companies, etc. Graymail hurts engagement metrics and can affect deliverability over time.Graylisting
A spam-fighting technique where a receiving server temporarily rejects an email from an unknown sender, expecting the legitimate sending server to retry later. Spammers often don’t retry. Graylisting causes a brief delivery delay (typically 5–30 minutes) for first-time senders. Lettr handles retries automatically.H
Hard Bounce
A permanent delivery failure indicating the email can never be delivered to that address. Common causes: the address doesn’t exist (SMTP 550), the domain doesn’t exist, or the mailbox has been disabled. Hard bounces should be immediately suppressed to protect sender reputation. See Bounce Diagnosis.Header
Metadata at the top of an email message containing routing information, authentication results, and display information (From, To, Subject, Date, Message-ID, etc.). Headers are normally hidden from the recipient but can be viewed in most email clients via “Show Original” or “View Source.” See Email Headers Explained.I
Idempotency
The property of an API operation where making the same request multiple times produces the same result as making it once. Lettr supports idempotent email sending via anIdempotency-Key header, preventing duplicate emails if a request is retried due to a network error.
See Idempotency.
IMAP (Internet Message Access Protocol)
A protocol used by email clients to retrieve and manage email stored on a mail server. Unlike SMTP, which handles sending, IMAP handles reading. IMAP keeps messages on the server and syncs across multiple devices. POP3 is an older alternative that downloads messages and typically removes them from the server.Implied Consent
Under CASL, consent inferred from an existing business relationship (such as a purchase or inquiry) without an explicit opt-in. Implied consent is time-limited — it expires 2 years after the last purchase or transaction, or 6 months after an inquiry. After expiration, express consent is required to continue sending. See CASL (Canadian Anti-Spam Law) and Email Consent Best Practices.Inbox Placement
Whether an email appears in the recipient’s primary inbox, as opposed to the spam folder, promotions tab, or other filtered location. Inbox placement is determined by sender reputation, authentication, content quality, and recipient engagement.Initial Open
The first time a recipient opens an email, tracked as a distinct event from subsequent opens. Initial open metrics provide a more accurate measure of unique engagement than total opens (which count every time the tracking pixel loads). Lettr firesinitial_open webhook events separately from generic open events.
See Event Types and Tracking Accuracy.
Injection
The first event in an email’s lifecycle, occurring when Lettr accepts an email for delivery via API or SMTP and queues it for transmission. A successful injection indicates the email passed validation and has been accepted into the sending pipeline. Injection events are logged and can trigger webhooks. Distinct from delivery (when the recipient server accepts the message) and inbox placement (when the email reaches the inbox). See Event Types and Webhook Event Types.Inline CSS
CSS styling applied directly to HTML elements using thestyle attribute rather than in a <style> block or external stylesheet. Inline CSS is the most reliable styling method for HTML email because some clients (notably Gmail on the web) strip <style> tags from the <head>. Many email tools automatically inline CSS during the build process.
See Email Content Rendering Issues and Email Rendering Across Clients.
IP Pool
A group of IP addresses managed by an ESP and shared across multiple senders. The ESP distributes outbound email across the pool and monitors reputation for each IP. IP pools allow ESPs to isolate senders by quality — high-quality senders share IPs with other high-quality senders. See Dedicated vs Shared IPs.IP Reputation
A score assigned to an IP address based on the quality and history of email sent from it. Mailbox providers track bounce rates, complaint rates, spam trap hits, and engagement from each IP. A poor IP reputation leads to spam filtering or rejection. See Sending Reputation.J
JMRP (Junk Mail Reporting Program)
Microsoft’s feedback loop program that sends notifications when Outlook.com and Hotmail users mark email as junk. JMRP is the Microsoft equivalent of a traditional FBL and provides complaint data that ESPs use to suppress complaining recipients. See Outlook / Microsoft 365 Delivery Issues and What Are Feedback Loops?.L
List Hygiene
The practice of maintaining a clean, valid email list by removing invalid addresses, handling bounces, processing complaints, re-engaging or sunsetting inactive subscribers, and validating addresses at collection. Poor list hygiene is the most common cause of deliverability problems. See List Hygiene.List-Unsubscribe Header
An email header that provides a machine-readable unsubscribe mechanism. Google and Yahoo require theList-Unsubscribe-Post header (RFC 8058) for one-click unsubscribe in bulk email. Lettr adds this header automatically when unsubscribe tracking is enabled.
See Unsubscribe Best Practices.
List Unsubscribe
An unsubscribe triggered by the recipient using the mailbox provider’s native unsubscribe interface (e.g., Gmail’s “Unsubscribe” button) via theList-Unsubscribe-Post header. Distinct from link unsubscribe, which occurs when a recipient clicks an unsubscribe link embedded in the email content. List unsubscribes are processed through one-click unsubscribe and may not load your preference center.
See Unsubscribe Best Practices and Webhook Event Types.
Loop Block
A special template block in Lettr that repeats for each item in an array passed as substitution data. Loop blocks enable dynamic list rendering (order items, feature lists, product catalogs) without manually creating individual blocks for each item. Loop blocks use a{{ #each items }}...{{ /each }} syntax and can access array properties using merge tags.
See Loop Blocks and Template Language.
M
MAIL FROM
The command in an SMTP conversation that specifies the envelope sender address (also called the return-path or bounce address). This is the address that receives bounce notifications and is checked by SPF. It can differ from the “From” header the recipient sees.Mailbox Provider
A service that operates recipient email inboxes — Gmail, Outlook.com, Yahoo Mail, Apple Mail (iCloud), and similar. Mailbox providers receive inbound email, apply spam filtering, and determine inbox placement. Also loosely referred to as “ISPs” (internet service providers) in the email industry, though this is technically imprecise. See What Is an Email Service Provider?.Mail Privacy Protection (MPP)
An Apple privacy feature (iOS 15+, macOS Monterey+) that pre-fetches email content — including tracking pixels — through Apple’s proxy servers at the time of delivery rather than when the recipient opens the email. This causes all emails to appear as “opened,” making open tracking unreliable for Apple Mail users. See Tracking Accuracy and Email Rendering Across Clients.Marketing Email
Email sent to promote a product, service, or offer. Marketing email requires explicit recipient consent (opt-in), must include an unsubscribe mechanism and physical address, and is subject to CAN-SPAM, GDPR, and other regulations. Should be sent separately from transactional email to protect transactional reputation. See Transactional vs Marketing.MCP (Model Context Protocol)
An open protocol for connecting AI assistants (like Claude) to external data sources and tools. Lettr provides two MCP servers: a remote server hosted athttps://app.lettr.com/mcp with OAuth authentication for chat-based AI clients, and a local server (lettr-mcp npm package) with API key authentication for developer tools. Both expose email sending, template management, and domain configuration as tools that AI assistants can invoke on behalf of users.
See MCP Introduction, Remote Server, Local Server, and Tools Reference.
Merge Tag
A placeholder variable in an email template (e.g.,{{ name }}, {{ order_id }}) that is replaced with recipient-specific data at send time. Merge tags enable personalization without creating individual templates for each recipient. The actual values that populate merge tags are provided as substitution data in API requests.
See Personalization and Dynamic Content and Template Language.
Message Threading
The email client feature that groups related messages into conversation threads using email headers (Message-ID, In-Reply-To, References). When replying to an email, the reply includes these headers to indicate the relationship to the original message. Lettr supports message threading for inbound emails, enabling reply tracking and conversation reconstruction.
See Reply Tracking and Email Headers Explained.
Message-ID
A unique identifier assigned to every email, formatted as<unique-string@domain>. Used for threading, deduplication, and troubleshooting. Each email should have exactly one Message-ID, generated by the sending system.
See Email Headers Explained.
MIME (Multipurpose Internet Mail Extensions)
A standard that extends email to support content beyond plain ASCII text — HTML, images, attachments, and multiple content types in a single message. MIME defines content types and encoding methods. Common multipart types include:multipart/alternative: Contains multiple versions of the same content (text + HTML)multipart/mixed: Contains message content plus attachmentsmultipart/related: Contains HTML content with inline images referenced via CID
MUA (Mail User Agent)
Client software used by end users to read, compose, and send email — such as Outlook, Apple Mail, Thunderbird, or webmail interfaces like Gmail. The MUA interacts with mail servers via IMAP/POP3 (for receiving) and SMTP (for sending). Contrast with MTA (Mail Transfer Agent), which is server software that routes email between mail servers. See How Email Delivery Works.MTA (Mail Transfer Agent)
Software responsible for sending, receiving, and routing email between servers using the SMTP protocol. Examples include Postfix, Sendmail, and Microsoft Exchange. When you send email through Lettr, Lettr’s MTA handles delivery to the recipient’s MTA.Multipart Message
An email that contains multiple content versions (typicallytext/plain and text/html) wrapped in a multipart/alternative MIME container. The recipient’s email client chooses which version to display based on its capabilities and user preferences. Sending multipart messages is a best practice for accessibility and deliverability.
See Email Content Types.
MX Record (Mail Exchanger)
A DNS record that specifies which mail server should receive email for a domain. MX records include a priority value — lower numbers indicate higher priority. When sending email, the sending server queries the recipient domain’s MX records to find the correct destination.O
One-Click Unsubscribe
An unsubscribe mechanism defined by RFC 8058 that allows mailbox providers to show an unsubscribe button in their UI. It requires theList-Unsubscribe and List-Unsubscribe-Post headers. Since February 2024, Google and Yahoo require one-click unsubscribe for bulk senders.
See Unsubscribe Best Practices and Google & Yahoo Sender Requirements.
Open Tracking
A technique that detects when a recipient opens an email by embedding an invisible tracking pixel (a 1x1 image). When the email client loads the image, the open is recorded. Open tracking is unreliable because images may be blocked, pre-fetched by privacy features (Apple Mail Privacy Protection), or loaded by security scanners. See Tracking Accuracy.Opt-In
The act of a recipient giving permission to receive email. Opt-in can be single (immediate subscription after form submission) or double (requires clicking a confirmation link in a verification email). GDPR and CASL require opt-in before sending commercial email, while CAN-SPAM uses an opt-out model. See Email Consent Best Practices.Opt-Out
The act of a recipient withdrawing permission to receive email, typically by clicking an unsubscribe link. Under CAN-SPAM, opt-out requests must be honored within 10 business days. Under GDPR and CASL, opt-out must be processed promptly. The opt-out mechanism must be clear, functional, and free. See Unsubscribe Best Practices.Out of Band Bounce
A bounce notification received separately from the original SMTP conversation, typically through a feedback loop or delayed DSN (Delivery Status Notification). Out-of-band bounces occur when the initial delivery succeeds but the message is later rejected by content filtering, mailbox rules, or delayed quota checks. These bounces are harder to associate with the original message. See Event Types.P
Phishing
A social engineering attack where a fraudulent email impersonates a trusted sender to trick recipients into revealing sensitive information or clicking malicious links. SPF, DKIM, and DMARC help prevent attackers from spoofing your domain in phishing emails.Policy Rejection
When an email is rejected by the recipient’s mail server due to policy violations rather than technical delivery issues. Policy rejections include: failing DMARC checks, sender IP/domain blocklisting, content filtering rules, or recipient-side anti-spam policies. Unlike hard bounces (invalid address), policy rejections indicate the recipient exists but the server refuses the message based on filtering rules. See Event Types and Bounce Diagnosis.Plus Addressing
A feature supported by most mailbox providers where a+ sign and additional text can be appended to the local part of an email address (e.g., user+newsletter@gmail.com). The email is delivered to the same mailbox as user@gmail.com. Plus addressing (also called sub-addressing or detailed addressing) is commonly used for filtering and identifying how an address was shared.
See Email Address Anatomy and Variable Reply-To Address.
Preheader Text
The short text snippet displayed next to or below the subject line in the inbox preview. Preheader text is extracted from the first text content in the HTML body, often hidden using CSS (display:none, font-size:0, or similar). Maximum length varies by client (typically 50-140 characters). You can set it explicitly using a hidden HTML element to provide additional context that encourages recipients to open the email, rather than showing template boilerplate.
Preference Center
A web page where recipients can manage their email subscription settings — choosing which types of email they want to receive rather than unsubscribing entirely. Preference centers reduce unsubscribes and complaints by giving recipients control. See Unsubscribe Best Practices.Premade Template
A professionally designed email template provided by Lettr in the template library. Premade templates cover common use cases (welcome emails, password resets, order confirmations, newsletters) and can be customized in the Topol editor. Using premade templates accelerates development and ensures responsive design best practices. See Premade Templates and Templates Introduction.Project
An organizational container in Lettr that groups related email templates into folders. Projects help teams organize templates by application, feature, or campaign type (e.g., “User Authentication,” “E-commerce,” “Notifications”). Templates within a project can share settings, and projects can have their own access controls in team environments. See Projects & Folders.Promotions Tab
Gmail’s tab for categorizing promotional and marketing emails separately from the Primary inbox. Emails placed in the Promotions tab are delivered successfully but receive less visibility. Tab placement is determined by Gmail’s machine learning classifier based on content style, sender relationship, engagement history, and the presence of an unsubscribe header. See Gmail-Specific Delivery Issues.PTR Record (Pointer Record)
A DNS record used for reverse DNS lookups — mapping an IP address back to a domain name. Mailbox providers check PTR records to verify that a sending IP address has a valid reverse DNS entry. Missing or misconfigured PTR records can cause email to be rejected or flagged as spam. See Google & Yahoo Sender Requirements.Q
Quoted-Printable Encoding
A MIME encoding method (defined in RFC 2045) that represents 8-bit characters using ASCII-safe=XX hexadecimal codes. Quoted-printable encoding is used for email content that is mostly ASCII text with occasional non-ASCII characters (accented letters, special symbols). It’s more human-readable than Base64 encoding and is the default for text/plain and text/html email parts.
See Email Parsing and Content Types.
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Rate Limiting
A server’s restriction on how many requests or emails a sender can submit within a given time period. Lettr enforces rate limits to protect infrastructure and deliverability (standard: 300 requests per 5 minutes). Exceeding limits returns a 429 HTTP status code with aretry_after value.
See Rate Limits and Rate Limits.
RCPT TO
An SMTP command that specifies one or more envelope recipients during the email transmission conversation. TheRCPT TO addresses can differ from the To: header visible to recipients — this is how BCC recipients and mailing list forwarding work. Multiple RCPT TO commands can be issued for a single message to deliver to multiple recipients.
See SMTP Protocol Basics.
Relay
The process of receiving inbound email on a domain configured with Lettr and forwarding (relaying) it to a destination endpoint via webhook or SMTP. Relay events track the lifecycle of inbound messages: injection (received), delivery (successfully forwarded), rejection (blocked by spam filtering or routing rules), temporary failure, and permanent failure. See Receiving Email Setup and Email Routing.Rendering Engine
The software component within an email client that interprets and displays HTML and CSS. Different clients use different rendering engines — Apple Mail uses WebKit, Outlook desktop uses Microsoft Word’s HTML engine, and Gmail uses a custom engine that strips<style> blocks. These differences are the primary cause of inconsistent email rendering across clients.
See Email Rendering Across Clients and Email Content Rendering Issues.
Reply-To Header
An email header that specifies where replies should be sent, which can differ from the “From” address. Useful when the sending address is a no-reply address but you want replies to go to a support or sales inbox.Reputation Isolation
The practice of using separate subdomains or IP addresses for different types of email (transactional, marketing, notifications) so that deliverability problems in one stream do not affect others. Reputation isolation protects high-priority email (like password resets) from being impacted by marketing campaign issues. See Subdomain vs Root Domain and Transactional vs Marketing.Return-Path
See Envelope From (Return-Path).Reverse DNS
A DNS lookup that resolves an IP address to a domain name (the opposite of a standard DNS lookup). Receiving mail servers perform reverse DNS lookups to verify that the sending server’s IP address has a valid hostname. A missing or mismatched reverse DNS entry is a common cause of delivery failures. See Google & Yahoo Sender Requirements.Role-Based Address
A generic email address associated with a function rather than an individual person (e.g.,admin@, support@, info@, sales@, postmaster@). Role-based addresses often route to shared inboxes and have higher complaint rates. Many ESPs recommend suppressing role-based addresses from marketing sends.
See List Hygiene.
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Sandbox Mode
A testing environment that allows you to verify your email integration without sending to real recipients or affecting your sending reputation. In Lettr’s sandbox mode, the API accepts requests and returns success responses, but emails are logged rather than transmitted to recipients. Sandbox mode is used during development and CI/CD testing to validate API requests, check response formats, and test webhook handling before going to production. See Sandbox and Testing Mode.Saved Block
A reusable content block template that can be saved from the Topol email editor and inserted into multiple email templates. Saved blocks are useful for standardizing common email components like headers, footers, social media icons, promotional banners, or legal disclaimers across templates. Changes to saved blocks do not automatically update templates where they’ve been inserted — they function as starting points rather than linked components. See Saved Blocks and Email Editor Best Practices.Seed List
A list of test email addresses across multiple mailbox providers used to verify inbox placement. Sending to seed lists helps diagnose deliverability issues by checking whether emails land in the inbox or spam folder at Gmail, Outlook, Yahoo, and other providers.Sender Reputation
A score or classification assigned to your sending domain and/or IP address by mailbox providers, based on historical sending behavior. Factors include bounce rate, complaint rate, spam trap hits, engagement, and sending consistency. Reputation is the single biggest factor in deliverability. See Sending Reputation.Shared IP
An IP address used by multiple senders on the same email platform. Your deliverability is partially influenced by the behavior of other senders sharing the IP. Shared IPs are the default for most email platforms (including Lettr) and work well for most sending volumes. See Dedicated vs Shared IPs.Single Opt-In
A subscription method where a recipient is immediately added to a mailing list after submitting a signup form, without a confirmation step. Single opt-in has lower friction and higher list growth rates but results in lower list quality (typos, bots, invalid addresses) and weaker consent evidence compared to double opt-in. See Email Consent Best Practices.SmartScreen
Microsoft’s content-based email filtering system that uses machine learning to classify messages as legitimate or spam. SmartScreen operates as a layer within Microsoft’s filtering stack alongside Exchange Online Protection and user-level signals (Safe Senders, block lists). See Outlook / Microsoft 365 Delivery Issues.SMTP (Simple Mail Transfer Protocol)
The standard protocol for transmitting email between servers. SMTP operates on port 25 (server-to-server), port 587 (submission with STARTTLS), and port 465 (submission with implicit TLS). SMTP defines the conversation between sending and receiving servers, including commands like EHLO, MAIL FROM, RCPT TO, and DATA. See SMTP Protocol Basics.SMTP Relay
A method of sending email where your application connects to an ESP’s SMTP server and transmits messages using the SMTP protocol. The ESP then relays the message to the recipient’s mail server. SMTP relay works with any language or tool that supports SMTP, making it compatible with legacy systems. Compare with sending via a REST API. See Sending via SMTP vs API.SNDS (Smart Network Data Services)
Microsoft’s service for monitoring the reputation of your sending IP addresses with Outlook.com and Microsoft 365. SNDS provides traffic volume, spam complaint rate, and a color-coded filter result (Green/Yellow/Red) per IP. It is the Microsoft equivalent of Google Postmaster Tools. See Outlook / Microsoft 365 Delivery Issues.Soft Bounce
A temporary delivery failure indicating the email could not be delivered right now but may succeed later. Common causes: mailbox full (SMTP 452), server temporarily unavailable (421), or message too large (552). Soft bounces are retried automatically. Persistent soft bounces may be reclassified as hard bounces. See Bounce Diagnosis.Spam Score
A numeric value (0.0–10.0) assigned to incoming emails indicating the likelihood of spam. Scores near 0.0 indicate legitimate email, while scores approaching 10.0 indicate almost certain spam. Lettr’s inbound email processing uses spam scores to route or filter messages based on configurable thresholds. See Spam Filtering.Spam Trap
An email address used by mailbox providers and blocklist operators to identify senders with poor list practices. Pristine traps are addresses that never opted in — hitting one means you’re using a purchased or scraped list. Recycled traps are abandoned addresses repurposed as traps — hitting one means you’re not cleaning inactive subscribers. See List Hygiene.Structure
In the Topol email editor, a structure is a layout container that defines column arrangement and responsive behavior for email content. Structures are added first when building a template, then populated with content blocks. Common structures include single-column, two-column, three-column, and asymmetric layouts. Structures handle responsive email design by automatically stacking columns on mobile devices. See Topol Email Editor and Email Editor Best Practices.Substitution Data
Variables and values passed in an API request that are merged into an email template at send time. Substitution data populates merge tags (e.g.,{{ first_name }}, {{ order_id }}) with recipient-specific information. Substitution data is processed per-recipient and should never contain sensitive information like passwords or credit card numbers. Distinct from metadata, which is used for tracking and analytics rather than content personalization.
See Template Language, Personalization, and Merge Tag.
SPF (Sender Policy Framework)
An email authentication protocol that allows domain owners to declare which mail servers are authorized to send email on their behalf. SPF is published as a DNS TXT record. Receiving servers check the sending server’s IP against the SPF record. SPF does not survive email forwarding. See Understanding SPF, DKIM, and DMARC.Spoofing
The act of sending email that falsely claims to be from a domain or address that the sender does not control. Spoofing is the foundation of phishing attacks. SPF, DKIM, and DMARC work together to detect and prevent domain spoofing. See Understanding SPF, DKIM, and DMARC.STARTTLS
An SMTP command that upgrades an existing plaintext connection to an encrypted TLS connection. After the TLS handshake completes, the SMTP session restarts over the encrypted channel. STARTTLS is used on port 587 and is the most common method for encrypting email in transit. See SMTP Protocol Basics.Subdomain
A prefix added to a root domain (e.g.,mail.yourdomain.com is a subdomain of yourdomain.com). Using subdomains for email sending provides reputation isolation — deliverability issues on one subdomain don’t affect others or your root domain.
See Subdomain vs Root Domain.
Sunset Policy
A rule that automatically removes or suppresses recipients who haven’t engaged with your email over a defined period (e.g., no opens or clicks in 90 days). Sunset policies prevent list decay, reduce complaints from disengaged recipients, and protect sender reputation. See List Hygiene and Sending Reputation.Suppression List
A list of email addresses that Lettr will not send to, regardless of whether they appear in a send request. Addresses are added automatically (hard bounces, complaints, unsubscribes) or manually. Suppression lists protect sender reputation by preventing email to known-bad addresses. See Understanding Suppression Lists.T
Tag
A single string label (maximum 64 characters) attached to an email at send time to categorize messages for analytics, filtering, and reporting. Unlike metadata (which stores key-value pairs for tracking), tags are used exclusively for grouping emails in the Lettr dashboard and analytics breakdowns. Multiple emails can share the same tag. See Email Tags and Analytics Filtering.Template Slug
A unique string identifier used to retrieve email templates via the Lettr API. Template slugs are URL-safe identifiers (typically lowercase with hyphens, e.g.,password-reset-email) that remain stable even when template names or content change. Using slugs instead of numeric IDs improves code readability and decouples code from template database IDs.
See Templates Introduction and Type Safety.
Template Version
A snapshot of an email template at a specific point in time. Lettr automatically creates versions when templates are edited, providing version control and rollback capability. Each version stores the complete template content, settings, and merge tags. Users can view version history, compare changes, and restore previous versions without losing the current version. See Template Versions and Templates Introduction.Throttling
When a mailbox provider intentionally slows down the rate at which it accepts email from a sender, typically in response to volume spikes, poor reputation, or warm-up concerns. Throttling results in deferrals (4xx responses) rather than outright rejections. The sending server is expected to retry later at a reduced rate. See Bounce Codes Reference and Batch and Bulk Sending.TLS (Transport Layer Security)
An encryption protocol that secures the connection between mail servers during SMTP transmission. TLS prevents eavesdropping on email content in transit. Most modern mail servers support opportunistic TLS (encrypt if both sides support it, fall back to unencrypted if not).Topol Email Editor
A drag-and-drop visual email editor integrated into Lettr for creating and editing HTML email templates without writing code. Topol uses a block-based system where users add content blocks (text, images, buttons, dividers) within structure containers (single column, multi-column layouts) that define responsive behavior. Templates created in Topol can include merge tags for personalization. See Topol Email Editor and Email Editor Best Practices.Tracking Domain
A custom domain used for open and click tracking instead of Lettr’s default tracking domain. Using a tracking domain that matches your sending domain improves deliverability and brand consistency. Tracking domains require CNAME records pointing to Lettr’s tracking infrastructure. See Tracking Domains.Tracking Pixel
A tiny (1x1 pixel) invisible image embedded in an HTML email to detect opens. When the recipient’s email client loads the image, the open event is recorded. Tracking pixels are the standard open-tracking method but are blocked by some email clients and privacy features. See Tracking Accuracy.Transactional Email
Email triggered by a user action or system event — password resets, order confirmations, shipping notifications, account alerts, and similar. Transactional email is expected by the recipient and typically has high open rates. It is generally exempt from CAN-SPAM’s unsubscribe requirements but must still include accurate sender information. See What Is Transactional Email? and Transactional vs Marketing.TXT Record
A DNS record type used to store text data. Email authentication relies heavily on TXT records: SPF policies, DKIM public keys, and DMARC policies are all published as TXT records.V
Variable Reply-To Address
A technique where data is encoded in the reply-to email address (e.g.,reply+ticket_ID@domain.com) to track which emails recipients reply to without parsing email headers. When a recipient replies, the variable portion is extracted from the recipient address, associating the reply with the original context. This is a more reliable alternative to threading headers for tracking replies across systems.
See Reply Tracking, Inbound Domains, and Plus Addressing.
Verified Mark Certificate (VMC)
A digital certificate issued by an authorized Certificate Authority (such as DigiCert or Entrust) that proves ownership of a trademarked logo. VMCs are required by Gmail and Apple Mail for displaying BIMI logos in recipient inboxes. Obtaining a VMC requires a registered trademark in an eligible jurisdiction. See BIMI: Brand Indicators for Message Identification.VERP (Variable Envelope Return Path)
A technique that encodes the recipient address in the envelope sender (return-path) address to precisely identify which recipient caused a bounce. Instead of using a generic bounce address likebounces@domain.com, VERP creates unique addresses like bounces+user=example.com@domain.com. When a bounce occurs, the recipient can be identified from the bounce address alone, without parsing bounce message content.
See Custom Return-Path.