Introduction

​

Popular Topics

​

Browse by Category

​

Concepts

• What Is Email Deliverability? — Deliverability vs delivery, and the factors that determine inbox placement
• What Is Transactional Email? — What makes email transactional, common types, and best practices
• What Are Feedback Loops? — How ISPs report spam complaints and how Lettr processes them
• What Is an Email Service Provider? — The role of an ESP and why you need one
• Dedicated vs Shared IPs — Pros, cons, and when to use each option

​

Email Fundamentals

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

​

Best Practices

• Deliverability — Maximize inbox placement
• Sending Reputation — Build and protect your sender reputation
• List Hygiene — Maintain healthy recipient lists
• Security — Protect your account and API keys
• Transactional vs Marketing — Understand the differences and why separation matters
• IP and Domain Warm-Up — Detailed warm-up schedules and volume ramp-up strategies
• Sending via SMTP vs API — Choose the right sending method for your use case
• Batch and Bulk Sending — Best practices for high-volume sending
• Email Accessibility — Alt text, semantic HTML, color contrast, and screen reader compatibility
• Dark Mode Email Design — Defensive CSS strategies for dark mode rendering
• Personalization and Dynamic Content — Merge tags, conditional blocks, and dynamic sections

​

Compliance & Regulations

• Google & Yahoo Sender Requirements — 2024 bulk sender requirements
• CAN-SPAM Requirements — US commercial email law compliance
• GDPR and Email Sending — EU data protection and consent requirements
• Unsubscribe Best Practices — Implementing effective unsubscribe mechanisms
• Data Privacy in Email Metadata — Protecting sensitive information
• CASL (Canadian Anti-Spam Law) — Canadian consent requirements and penalties
• Email Consent Best Practices — Obtaining, recording, and managing email consent
• Acceptable Use Policy — What you can and cannot send, prohibited content, and anti-abuse measures
• Data Retention and Deletion — How long data is retained and how to handle deletion requests

​

Troubleshooting

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

​

DNS Guides

• AWS Route 53
• Cloudflare
• GoDaddy
• Hetzner
• Vercel

​

Use Cases / Recipes

• Password Reset Emails — Secure token generation, email content, and delivery speed optimization
• Order Confirmation Emails — Structure, required information, delivery timing, and template design
• Welcome and Onboarding Emails — Sequence design, timing, and transactional vs marketing classification
• Two-Factor Authentication Emails — OTP delivery, security considerations, and rate limiting
• Invoice and Billing Emails — Legal requirements, formatting, attachments, and dunning sequences

​

Glossary

• Email Glossary — A–Z reference of email industry terms, protocols, and concepts

​

Quick Answers

​

Need More Help?