5xx reply
An SMTP reply starting with 5, a permanent failure: the server refused the step and the sender should not repeat it unchanged.
What it means
RFC 5321 calls these permanent negative completion replies. The most common is 550, used both for a mailbox that does not exist and for policy refusals; 552 (storage or size exceeded), 553 (mailbox name not allowed) and 554 (transaction failed) are frequent too, and 500 to 504 report errors in the commands themselves. The enhanced status code that follows usually tells the cases apart: 5.1.1 for an unknown address, 5.7.x for policy and authentication.
A 5xx reply ends delivery to that recipient at once and produces a bounce. The reason decides what to do next: a 5.1.1 address will never work and should be suppressed, while a 5.7.1 block concerns the sender and is not fixed by removing the address. Yahoo asks senders not to retry 5xx failures, and list managers to remove addresses that produce them.
Related 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
- Yahoo Sender Hub: SMTP error codessenders.yahooinc.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes