Mail loop
A message passed around in a circle between servers or forwarding rules, which servers detect by counting Received headers and stop with a bounce.
What it means
Loops come from configuration: two forwarding rules pointing at each other, an MX host that relays mail back toward its origin, an auto-reply answering another auto-reply. RFC 5321 notes that counting Received headers is a simple and effective way to detect them, and suggests a high threshold, normally at least 100. Servers that hit their limit stop the message with an error such as 5.4.6, routing loop detected; Gmail returns 554 5.4.6, Microsoft reports hop count exceeded with 554 5.4.14, and Exchange breaks a loop after 20 iterations by default.
Automatic replies are the other classic loop, which is why RFC 3834 tells responders not to answer messages marked Auto-Submitted and never to reply to a null sender.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 6.3: Loop Detectionrfc-editor.org
- RFC 3834, section 2: When (not) to send automatic responsesrfc-editor.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
- 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