4xx reply
An SMTP reply starting with 4, a temporary failure: the server did not accept the step now, and the sender should keep the message and try again later.
What it means
RFC 5321 calls these transient negative completion replies, and gives a rule of thumb: a reply is 4xx if the same command could succeed when repeated unchanged. Common ones are 421 (service not available, often a rate limit or a closing connection), 450 (mailbox unavailable for now), 451 (local error, used for greylisting and many throttles) and 452 (insufficient storage, or too many recipients).
The sending server keeps the message in its queue and retries on a schedule, usually for several days, before giving up and returning a bounce. Large receivers use 4xx replies to slow senders down: Gmail's 421 4.7.28 and Yahoo's TS deferrals are examples. Google's advice when deferrals appear is to reduce sending volume until the error rate falls, then increase it slowly.
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
- Google, Gmail Help: Email sender guidelinessupport.google.com
- Yahoo Sender Hub: SMTP error codessenders.yahooinc.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes