The SMTPTester Blog
Practical, no-fluff guides on SMTP, authentication, encryption and email deliverability. Written by engineers who debug this stuff for a living.
How to Test an SMTP Server: Tools, Commands and a Full Diagnostic Workflow
A complete guide to testing SMTP servers — online testing, telnet and openssl commands, TLS verification, authentication checks and a repeatable diagnostic workflow.
Email Bounce Handling: Hard vs Soft Bounces and How to Process Them
How bounces work, the difference between hard and soft bounces, how to parse DSN messages, build a suppression list, and keep bounce rates under control.
Microsoft 365 SMTP Setup: Authenticated Submission, Relay and Connectors
How to send email through Microsoft 365 — SMTP AUTH settings, enabling authenticated SMTP, direct send, high-volume connectors, OAuth, limits and error fixes.
Gmail SMTP Setup Guide: Settings, App Passwords, Limits and Fixes
Complete Gmail and Google Workspace SMTP configuration — server settings, app passwords, OAuth 2.0, sending limits, relay options and every common error solved.
SMTP Ports Explained: 25, 465, 587 and 2525 — Which One Should You Use?
A complete guide to SMTP port selection: what each port is for, how STARTTLS differs from implicit TLS, why port 25 is blocked, and how to diagnose firewall problems.
The Complete Email Deliverability Checklist for 2025
Everything that decides whether your email reaches the inbox — authentication, infrastructure, list hygiene, content, engagement and monitoring — in one actionable checklist.
Every SMTP Error Code Explained: A Complete Reference for Developers
What each SMTP reply code and enhanced status code actually means, which are temporary, which are permanent, and exactly how to fix the ones you will really meet.
SPF, DKIM and DMARC: The Complete Email Authentication Setup Guide
Step-by-step instructions for publishing SPF, DKIM and DMARC records correctly, understanding alignment, reading reports, and moving safely to p=reject.
SMTP vs API Email Sending: Which One Should You Use?
A pragmatic comparison of SMTP relay and HTTP API based email sending — performance, observability, deliverability, and when to pick each.
Best SMTP Settings for Reliable Email Delivery in 2025
The exact ports, encryption modes, authentication, and timeouts to use for production SMTP — plus a copy-paste config for the top providers.
How to Fix SMTP Authentication Errors (535, 530, 534)
A practical, copy-paste friendly guide to debugging SMTP authentication failures across Gmail, Microsoft 365, SendGrid, and self-hosted servers.