SMTP error codes can be confusing, especially when you're trying to understand why an email was delayed, bounced, or rejected.
I put together a guide covering common 4xx and 5xx SMTP error codes, what they usually indicate, and practical ways to troubleshoot them.
It covers issues such as:
- Temporary vs. permanent SMTP failures
- Mailbox unavailable and quota errors
- Authentication failures
- DNS and routing problems
- Policy and security rejections
- Common enhanced SMTP status codes
- Steps to diagnose and fix delivery issues
If you're working with email servers, SMTP infrastructure, or email deliverability, hopefully this can be a useful reference.
Full guide:
https://smtpmaster.com/blog/smtp-error-codes/
What SMTP error code do you encounter most often?
https://i.redd.it/3kz9hbh811rh1.png
Source: r/SMTPMaster · by /u/EmailSMTPMaster