Delivery timeout
How long a sending server keeps retrying a deferred message before it gives up and returns a bounce; RFC 5321 suggests at least four to five days.
What it means
A message that keeps meeting temporary failures cannot stay queued forever. RFC 5321 says the give-up time generally needs to be at least 4 to 5 days, and allows a shorter limit for bounce messages themselves. Postfix's default maximal_queue_lifetime is 5 days; other MTAs and sending platforms choose their own values, some much shorter for time-sensitive mail.
When the time runs out the sender returns a non-delivery report, usually with enhanced status code 4.4.7, delivery time expired (Microsoft reports it as 550 4.4.7), and quoting the last temporary error it received, which is the clue to what went wrong. Before that point many servers send a delay notification so the sender knows the message is stuck.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.5.4.1: Sending Strategyrfc-editor.org
- Postfix configuration parameters, postconf(5)postfix.org
- IANA, SMTP Enhanced Status Codes registryiana.org
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes