SMTP bounce codes tell you exactly why an email failed. Here's your complete reference guide.
Code Structure
Codes follow X.Y.Z format: X=class (4xx temp, 5xx perm), Y=subject, Z=detail.
Common Codes
- 550 5.1.1: User unknown (hard bounce)
- 552 5.2.2: Mailbox full (soft bounce)
- 421 4.7.0: Connection rate limited
- 550 5.7.1: Message rejected (spam)
How to Handle
4xx: Retry later. 5xx: Remove from list if user-related, or fix issue if server-related.
Conclusion
Understanding bounce codes helps you take the right action quickly.