> ## Documentation Index
> Fetch the complete documentation index at: https://docs.lettr.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

> The Lettr Knowledge Base: email fundamentals, deliverability best practices, troubleshooting, and compliance guides for reliable sending.

Find answers to common questions, learn email fundamentals, troubleshoot issues, and follow best practices for sending email with Lettr.

## Popular Topics

<CardGroup cols={2}>
  <Card title="Deliverability Best Practices" icon="inbox" href="/knowledge-base/best-practices/deliverability">
    Maximize inbox placement and maintain sender reputation
  </Card>

  <Card title="Emails Landing in Spam" icon="shield-exclamation" href="/knowledge-base/troubleshooting/spam-placement">
    Step-by-step diagnosis for spam folder issues
  </Card>

  <Card title="Google & Yahoo Requirements" icon="envelope-circle-check" href="/knowledge-base/compliance/google-yahoo-requirements">
    Meet the 2024 bulk sender requirements
  </Card>

  <Card title="How Email Delivery Works" icon="diagram-project" href="/knowledge-base/fundamentals/how-email-delivery-works">
    Understand the email delivery pipeline end to end
  </Card>
</CardGroup>

## Browse by Category

### Concepts

Foundational explanations of core email concepts.

* [What Is Email Deliverability?](/knowledge-base/concepts/email-deliverability) — Deliverability vs delivery, and the factors that determine inbox placement
* [What Is Transactional Email?](/knowledge-base/concepts/transactional-email) — What makes email transactional, common types, and best practices
* [What Are Feedback Loops?](/knowledge-base/concepts/feedback-loops) — How ISPs report spam complaints and how Lettr processes them
* [What Is an Email Service Provider?](/knowledge-base/concepts/email-service-provider) — The role of an ESP and why you need one
* [Dedicated vs Shared IPs](/knowledge-base/concepts/dedicated-vs-shared-ips) — Pros, cons, and when to use each option

### Email Fundamentals

Educational content explaining how email infrastructure works.

* [How Email Delivery Works](/knowledge-base/fundamentals/how-email-delivery-works) — End-to-end overview of the email delivery pipeline
* [Understanding SPF, DKIM, and DMARC](/knowledge-base/fundamentals/spf-dkim-dmarc) — How authentication protocols protect your domain
* [Email Headers Explained](/knowledge-base/fundamentals/email-headers) — What common headers mean and how they affect delivery
* [Bounce Codes Reference](/knowledge-base/fundamentals/bounce-codes-reference) — Comprehensive SMTP response code reference
* [Understanding Suppression Lists](/knowledge-base/fundamentals/suppression-lists) — What suppression lists are and how they work
* [Email Content Types](/knowledge-base/fundamentals/content-types) — HTML, plain text, and AMP email explained
* [Subdomain vs Root Domain](/knowledge-base/fundamentals/subdomain-vs-root) — Choosing the right domain strategy for sending
* [SMTP Protocol Basics](/knowledge-base/fundamentals/smtp-protocol-basics) — How SMTP works, common ports, TLS, and the SMTP conversation
* [Email Address Anatomy](/knowledge-base/fundamentals/email-address-anatomy) — Local part, domain, display name, plus addressing, and case sensitivity
* [BIMI](/knowledge-base/fundamentals/bimi) — Display your brand logo in recipient inboxes with Verified Mark Certificates
* [Custom Return-Path and MAIL FROM](/knowledge-base/fundamentals/custom-return-path) — How custom bounce domains improve SPF alignment
* [Email Rendering Across Clients](/knowledge-base/fundamentals/email-rendering-clients) — How Gmail, Outlook, Apple Mail, and others render HTML email differently

### Best Practices

Guidance for optimizing your email sending.

* [Deliverability](/knowledge-base/best-practices/deliverability) — Maximize inbox placement
* [Sending Reputation](/knowledge-base/best-practices/sending-reputation) — Build and protect your sender reputation
* [List Hygiene](/knowledge-base/best-practices/list-hygiene) — Maintain healthy recipient lists
* [Security](/knowledge-base/best-practices/security) — Protect your account and API keys
* [Transactional vs Marketing](/knowledge-base/best-practices/transactional-vs-marketing) — Understand the differences and why separation matters
* [IP and Domain Warm-Up](/knowledge-base/best-practices/ip-domain-warmup) — Detailed warm-up schedules and volume ramp-up strategies
* [Sending via SMTP vs API](/knowledge-base/best-practices/smtp-vs-api) — Choose the right sending method for your use case
* [Batch and Bulk Sending](/knowledge-base/best-practices/batch-bulk-sending) — Best practices for high-volume sending
* [Email Accessibility](/knowledge-base/best-practices/email-accessibility) — Alt text, semantic HTML, color contrast, and screen reader compatibility
* [Dark Mode Email Design](/knowledge-base/best-practices/dark-mode-design) — Defensive CSS strategies for dark mode rendering
* [Personalization and Dynamic Content](/knowledge-base/best-practices/personalization-dynamic-content) — Merge tags, conditional blocks, and dynamic sections

### Compliance & Regulations

Stay compliant with email regulations and provider requirements.

* [Google & Yahoo Sender Requirements](/knowledge-base/compliance/google-yahoo-requirements) — 2024 bulk sender requirements
* [CAN-SPAM Requirements](/knowledge-base/compliance/can-spam) — US commercial email law compliance
* [GDPR and Email Sending](/knowledge-base/compliance/gdpr-email) — EU data protection and consent requirements
* [Unsubscribe Best Practices](/knowledge-base/compliance/unsubscribe-best-practices) — Implementing effective unsubscribe mechanisms
* [Data Privacy in Email Metadata](/knowledge-base/compliance/data-privacy-metadata) — Protecting sensitive information
* [CASL (Canadian Anti-Spam Law)](/knowledge-base/compliance/casl) — Canadian consent requirements and penalties
* [Email Consent Best Practices](/knowledge-base/compliance/email-consent) — Obtaining, recording, and managing email consent
* [Acceptable Use Policy](/knowledge-base/compliance/acceptable-use-policy) — What you can and cannot send, prohibited content, and anti-abuse measures
* [Data Retention and Deletion](/knowledge-base/compliance/data-retention) — How long data is retained and how to handle deletion requests

### Troubleshooting

Diagnose and fix common issues.

* [Delivery Issues](/knowledge-base/troubleshooting/delivery-issues) — Why emails aren't being delivered
* [Emails Landing in Spam](/knowledge-base/troubleshooting/spam-placement) — Diagnose spam folder placement
* [Bounce Diagnosis](/knowledge-base/troubleshooting/bounce-diagnosis) — Interpret bounce codes and take action
* [Authentication Issues](/knowledge-base/troubleshooting/authentication) — API key and auth problems
* [Rate Limits](/knowledge-base/troubleshooting/rate-limits) — Understanding and handling rate limits
* [Domain Verification](/knowledge-base/troubleshooting/domain-verification) — Fix DNS and verification failures
* [Tracking Accuracy](/knowledge-base/troubleshooting/tracking-accuracy) — Why open and click rates may be inaccurate
* [Gmail-Specific Delivery Issues](/knowledge-base/troubleshooting/gmail-delivery) — Diagnose Gmail tabs, Postmaster Tools, and common Gmail blocks
* [Outlook / Microsoft 365 Delivery Issues](/knowledge-base/troubleshooting/outlook-delivery) — SNDS, SmartScreen, and Outlook-specific blocks
* [Webhook Delivery Failures](/knowledge-base/troubleshooting/webhook-failures) — Retry logic, timeouts, and debugging failed webhook deliveries
* [Email Content Rendering Issues](/knowledge-base/troubleshooting/rendering-issues) — Broken layouts, missing images, font fallbacks, and Gmail clipping
* [Sandbox and Testing Mode](/knowledge-base/troubleshooting/sandbox-testing) — Test email sending without delivering to real recipients

### DNS Guides

Step-by-step DNS configuration for specific providers.

* [AWS Route 53](/knowledge-base/dns-guides/aws-route53)
* [Cloudflare](/knowledge-base/dns-guides/cloudflare)
* [GoDaddy](/knowledge-base/dns-guides/godaddy)
* [Hetzner](/knowledge-base/dns-guides/hetzner)
* [Vercel](/knowledge-base/dns-guides/vercel)

### Use Cases / Recipes

Practical guides for implementing common email workflows.

* [Password Reset Emails](/knowledge-base/use-cases/password-reset-emails) — Secure token generation, email content, and delivery speed optimization
* [Order Confirmation Emails](/knowledge-base/use-cases/order-confirmation-emails) — Structure, required information, delivery timing, and template design
* [Welcome and Onboarding Emails](/knowledge-base/use-cases/welcome-onboarding-emails) — Sequence design, timing, and transactional vs marketing classification
* [Two-Factor Authentication Emails](/knowledge-base/use-cases/two-factor-authentication-emails) — OTP delivery, security considerations, and rate limiting
* [Invoice and Billing Emails](/knowledge-base/use-cases/invoice-billing-emails) — Legal requirements, formatting, attachments, and dunning sequences

### Glossary

* [Email Glossary](/knowledge-base/glossary/email-glossary) — A–Z reference of email industry terms, protocols, and concepts

## Quick Answers

<AccordionGroup>
  <Accordion title="Why isn't my email being delivered?">
    Check your domain verification status, ensure the recipient isn't suppressed, and verify you're not hitting rate limits. See [Delivery Issues](/knowledge-base/troubleshooting/delivery-issues) for detailed troubleshooting.
  </Accordion>

  <Accordion title="Why are my emails going to spam?">
    Common causes include missing authentication (DKIM, DMARC), poor sender reputation, spam trigger words in content, or missing unsubscribe links. See [Emails Landing in Spam](/knowledge-base/troubleshooting/spam-placement) for a step-by-step diagnosis guide.
  </Accordion>

  <Accordion title="How do I verify my domain?">
    Add the DNS records shown in your dashboard (CNAME, DKIM, DMARC), wait for propagation, then click Verify. See [Domains](/learn/domains/introduction) for details or [Domain Verification](/knowledge-base/troubleshooting/domain-verification) if you're having issues.
  </Accordion>

  <Accordion title="What are the API rate limits?">
    Standard: 300 requests per 5 minutes. High-volume: 2,000 requests per 5 minutes. See [Rate Limits](/knowledge-base/troubleshooting/rate-limits) for details and handling strategies.
  </Accordion>

  <Accordion title="Do I need an unsubscribe link?">
    Marketing and promotional emails require an unsubscribe link by law (CAN-SPAM, GDPR). Since February 2024, Google and Yahoo also require one-click unsubscribe for bulk senders. See [Unsubscribe Best Practices](/knowledge-base/compliance/unsubscribe-best-practices).
  </Accordion>

  <Accordion title="Should I send from a subdomain or root domain?">
    For most senders, a subdomain (like `mail.yourdomain.com`) is recommended. It provides reputation isolation so problems with one email type don't affect others. See [Subdomain vs Root Domain](/knowledge-base/fundamentals/subdomain-vs-root).
  </Accordion>
</AccordionGroup>

## Need More Help?

<CardGroup cols={3}>
  <Card title="Documentation" icon="book" href="/introduction">
    Complete product documentation
  </Card>

  <Card title="API Reference" icon="code" href="/api-reference/introduction">
    Technical API documentation
  </Card>

  <Card title="Contact Support" icon="headset" href="mailto:support@lettr.com">
    Get help from our team
  </Card>
</CardGroup>