Delay notification
A warning sent to the sender while a message is still being retried, saying delivery is delayed but the server will keep trying; it is not a bounce.
What it means
A server holding a message in its queue for a long time may tell the sender before it gives up. In DSN terms this is a report with Action: delayed and a 4.x.x status, which may carry a Will-Retry-Until field giving the time the server will stop trying (RFC 3464). Senders can ask for such reports with NOTIFY=DELAY on RCPT TO, or rule them out with NOTIFY=NEVER.
Whether and when one is sent depends on the MTA: Postfix, for example, sends one only when delay_warning_time is set to a non-zero value, and it is zero by default. A delay notification needs no action yet, but the reason it quotes (a timeout, a 421 reply, greylisting) shows what is holding the message up.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 3464, section 2.3.3: Action fieldrfc-editor.org
- RFC 3464, section 2.3.9: Will-Retry-Until fieldrfc-editor.org
- RFC 3461, section 4.1: The NOTIFY parameter of the ESMTP RCPT commandrfc-editor.org
- Postfix configuration parameters, postconf(5)postfix.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes