SMTP rejection
A receiving server refusing a message during the SMTP session with a 5xx reply, so the sender knows at once and its own system writes the bounce.
What it means
A rejection can come at any step: at connection (a 554 greeting), after MAIL FROM, for an individual RCPT TO, or after the message data, which is when content and authentication checks usually run. Because the refusal happens while the sending server is still connected, nothing has been accepted, and the sender, not the receiver, is responsible for telling the author.
Rejecting during the session never sends mail to a forged address, which is why it is the recommended way to refuse unwanted mail: RFC 5321 discourages bouncing messages found to contain hostile content, and Postfix's backscatter guidance rests on rejecting during the session. For senders, rejections are the easiest failures to diagnose, since the exact reply is in their own logs.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 3.1: Session Initiationrfc-editor.org
- RFC 5321, section 6.2: Unwanted, Unsolicited, and "Attack" Messagesrfc-editor.org
- Postfix Backscatter Howtopostfix.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes