SMTP error codes and bounce messages explained
A bounce message carries two codes. The reply code is three digits, such as 550, and comes from RFC 5321. The enhanced status code has three parts, such as 5.1.1, and comes from RFC 3463. The first digit of the enhanced code tells you the kind of failure: 5.x.x is permanent (a hard bounce) and 4.x.x is temporary (a soft bounce).
Each page below says what one code means, what Gmail and Microsoft print for it, the usual causes, how to fix it, and whether it points to a deliverability problem on your side.
22 matching codes
Invalid recipient
Mailbox full
Blocked by the provider
- 421 4.7.0Temporarily deferred, often for low reputation
- 421 4.7.26Rate limited because mail is unauthenticated
- 421 4.7.28Unusual rate of unsolicited mail from your domain
- 550 5.7.1Delivery not authorized, message refused
- 554 5.7.1Relay access denied or client host rejected
- 550 5.7.23SPF validation failed
- 550 5.7.25Sending IP has no reverse DNS record
- 550 5.7.26Blocked because the sender is unauthenticated
- 550 5.7.27Blocked because SPF did not pass
- 550 5.7.509Sending domain fails DMARC with a reject policy
- 550 5.7.606Microsoft banned the sending IP
- 550 5.7.708Microsoft does not accept traffic from this IP
Temporary failure
Unrecognised
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