SMTP reply code
The three-digit number a mail server returns for every SMTP command, such as 250, 421 or 550, whose first digit says whether the step succeeded, failed for now or failed for good.
What it means
RFC 5321 gives each digit a role. The first is the outcome: 2 success, 3 waiting for more input, 4 temporary failure, 5 permanent failure. The second narrows the topic: x0x syntax, x1x information, x2x the connection, x5x the mail system. The third adds detail. A text explanation follows the number, and many servers put an enhanced status code such as 5.1.1 at the start of that text.
Sending software acts on the first digit: a 4xx reply puts the message back in the queue for another try, a 5xx reply ends the attempt for that recipient and produces a bounce. The text after the code is where receivers explain the reason, often with a link, so it is worth reading in full.
Example
550 5.1.1 <old.user@example.net>: Recipient address rejected: User unknownRelated terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.2.1: Reply Code Severities and Theoryrfc-editor.org
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes