📚 SMTP Guides
📚 References/Knowledge
Comprehensive SMTP technical documentation and implementation guides.
🤝 What Is SMTP? The Handshake Explained
HELO/EHLO, MAIL FROM, RCPT TO, DATA with transcript examples
🔌 SMTP Ports 25 vs 465 vs 587
When to Use Each and Why
🔒 STARTTLS vs Implicit TLS
How SMTP Encryption Really Works
🔐 SMTP Authentication Methods
LOGIN, PLAIN, CRAM-MD5, and OAuth2 with Code Samples
⚠️ SMTP Error Codes Lookup
Causes and Fixes for 421/450/451/452/534/535/550/551/552/554
📧 Gmail/Google Workspace SMTP
Settings, App Passwords, and Common Errors
🏢 Microsoft 365/Exchange Online SMTP
SMTP Auth, Connectors, and Troubleshooting
🌐 Yahoo/Zoho/Other Providers
Verified Settings and Limitations
🐧 Postfix SMTP Sending Guide
Main.cf Examples, TLS, Auth, and Logs
⚙️ Exim/cPanel SMTP Guide
Configuration, Ports, and Rate Limits
⏱️ Troubleshooting SMTP Timeouts
Firewall, ISP Blocks, and Port Reachability
🔏 TLS/Certificate Problems in SMTP
Expired Certs, Name Mismatch, and Chain Issues
🛡️ rDNS/PTR, SPF, DKIM, DMARC
How They Affect SMTP Accept/Reject Decisions
🐌 Greylisting, Throttling, and Rate Limits
Why Your Mail Is Delayed
📖 SMTP Glossary A–Z
Definitions + Internal Links to Guides
📋 Policy & Legal
Privacy, security, and usage policies for SMTP testing tools.
🔒 Privacy & Terms Essentials
Data Handling and User Rights
⚖️ Security & Usage Disclaimer
Responsible Use Guidelines