Enhanced status code
Also called: 5.1.1, extended status code
A three-part machine-readable code in the form class.subject.detail (for example 5.1.1) that accompanies an SMTP reply. The class is 2, 4 or 5, meaning success, transient failure or permanent failure.
In plain terms
The longer dotted number next to the reply code, giving a far more specific reason.
Why it matters for deliverability
The three-digit reply tells you whether to retry; the enhanced code tells you why. 5.1.1 (bad destination mailbox) and 5.7.1 (delivery not authorised) are both 550s, but one is a list-quality problem and the other is an authentication or policy problem.
What to do about it
- Parse and store the enhanced code alongside the reply code.
- Route 5.1.x to list hygiene and 5.7.x to authentication and policy review.
- Use the subject digit to group bounce reasons in reporting instead of grouping by provider.
Source
RFC 3463 — Enhanced Mail System Status Codes
“status-code = class "." subject "." detail class = "2"/"4"/"5"”
Checked 2026-08-29
Go deeper
Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
See how your own mail is handled
Run a real message through MailTester and read the authentication results, spam score and delivery signals described on this page against your own domain.