SMTP
SMTP protocol configuration, relay setup, and mail server basics.
20 articles
End of Basic Auth SMTP on Microsoft Exchange Online: timeline, 550 5.7.30 error, and migration guide
Microsoft is retiring Basic authentication for SMTP AUTH on Exchange Online. Revised timeline, 550 5.7.30 error, OAuth alternatives, HVE, Graph API, and step-by-step migration checklist.
Port 25 blocked: diagnosis and solutions by hosting provider
Port 25 is blocked by most cloud providers and ISPs to fight outbound spam. Here's how to diagnose the block and restore your sending capability.
STARTTLS, SSL/TLS and SMTP: which encryption for your emails?
SSL, TLS, STARTTLS, Implicit TLS, DANE, MTA-STS: email encryption in transit relies on mechanisms that are often confused. This guide clarifies each protocol, explains their vulnerabilities, and shows how to configure robust TLS encryption on Postfix, Exim and Exchange.
SMTP ports explained: 25, 465, 587, 2525, which one should you use?
Port 25, 465, 587, or 2525? Each SMTP port has a specific role. This guide breaks down how each port works, its encryption method, the relevant RFCs, and helps you pick the right port for your use case.
How to test the SMTP connectivity of your MX servers
Your DNS records are perfect, but emails aren't getting through? The problem might be at the transport layer. This guide shows you how to test SMTP connectivity for each MX server, step by step.
Postmark (ActiveCampaign): Complete Technical Guide to Transactional Email
Detailed technical guide for developers: DNS configuration (DKIM, custom Return-Path), REST API vs SMTP, Message Streams, dedicated IP, webhooks and 2026 pricing.
Mandrill (Mailchimp Transactional): Technical Guide for Transactional Email
A complete guide for developers and system admins: DKIM configuration via CNAME, choosing between API and SMTP, Mailchimp prerequisites, and pitfalls to avoid.
Mailgun: Complete Technical Guide for Transactional Email
A detailed technical guide for developers: DNS configuration (SPF, 2048-bit DKIM with autorotation), REST API vs SMTP, dedicated vs shared IP, limits and webhooks.
Amazon SES: Complete Guide to DNS Configuration and Email Authentication
Complete Amazon SES configuration with Easy DKIM, Custom MAIL FROM, strict DMARC alignment. Everything you need for optimal deliverability and GDPR compliance.
Mailjet: Complete Technical Guide for Transactional Email in 2026
A comprehensive guide for developers and system admins: DKIM configuration, API v3 vs v3.1, dedicated vs shared IP, and pitfalls to avoid with Mailjet.
Twilio SendGrid: Complete Technical Guide for Transactional Email
A detailed technical guide for developers: DNS configuration (SPF, 2048-bit DKIM), Web API v3, SMTP Relay, dedicated vs shared IP, limits and webhooks.
Brevo (formerly Sendinblue): Technical Guide for Transactional Emailing
A comprehensive guide for developers and sysadmins: DKIM configuration, choosing between API and SMTP, technical limits, and pitfalls to avoid with Brevo.