SMTP Enhanced Status Code

An SMTP enhanced status code is the dotted three-part code defined in RFC 3463, written as class.subject.detail, that a mail server places at the start of its reply text to say precisely what happened to a message. The 3-digit reply code that SMTP itself defines, such as 550, only tells a client whether to give up, retry, or carry on; the enhanced code alongside it, such as 5.1.1 (the destination mailbox does not exist) or 4.2.2 (mailbox full), tells software and administrators why. A server signals that it will send these codes by advertising the ENHANCEDSTATUSCODES keyword in its EHLO response, per RFC 2034. The same codes appear in the Status field of a DSN bounce report, and every registered code is listed in IANA’s SMTP Enhanced Status Codes registry, set up by RFC 5248.

How do you read the class and subject of an enhanced status code?

The class is the first digit and must agree with the reply code: 2 means success, 4 a persistent transient failure that may clear on retry, and 5 a permanent failure that resending the same message will not fix. The subject is the second number and names the area of the problem: 0 other or undefined, 1 addressing, 2 mailbox, 3 mail system, 4 network and routing, 5 mail delivery protocol, 6 message content or media, and 7 security or policy. The detail narrows it further within that subject. Many rejections caused by sender authentication or reputation checks fall under the 5.7.x security or policy subject. Large mail providers document the codes their own servers return: Microsoft’s Exchange Online NDR reference lists codes such as 5.1.1 and 5.7.1 with their likely causes in that service.

To decode a real bounce line, paste it into AdminKit’s SMTP status code decoder. SMTP enhanced status codes are part of AdminKit’s email authentication cluster; the full AdminKit glossary lists other related terms.