Asynchronous bounce
A bounce that arrives later as a separate email, because a server first accepted the message and only afterwards found it could not deliver it.
What it means
When a receiver refuses a message during the SMTP session, the sender learns the outcome at once from the reply code. When a server accepts first, with 250 after the data, and fails afterwards, RFC 5321 requires it to send a non-delivery notification to the envelope sender. The RFC names typical causes: a relay that cannot check recipients when it accepts, a mailing list target, or a temporary DNS problem.
Asynchronous bounces are harder to handle: they arrive minutes or days later, in varying formats, and go to whatever envelope sender the message carried, real or forged. That last point is the source of backscatter, and Postfix's guidance on backscatter starts from rejecting mail during the SMTP session instead of accepting it and bouncing later.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 6.1: Reliable Delivery and Replies by Emailrfc-editor.org
- Postfix Backscatter Howtopostfix.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes