5.4.6: mail routing loop
The message was passed back and forth between servers, or forwarded around a circle, until a server noticed the loop and stopped it.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.4.6
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The recipient's side. Their mail admin has to change something; you can tell them.
- What the server says
5.4.6 Routing loop detected
What it means
X.4.6 says the message was forwarded too many times, because routing tables point servers at each other or because forwarding rules send it around a circle (A forwards to B, B forwards back to A). RFC 5321 describes counting the Received header fields as an effective way to notice, with a high threshold. RFC 3463 calls the code useful only as a persistent temporary error (4.4.6), yet a loop seldom clears by itself, so in practice the stop is reported as a permanent 5.4.6 and the message is returned; Microsoft, for example, documents 554 5.4.6 from on-premises Exchange servers for a mail loop. It is a configuration problem for the domain owner, or for the mailbox owner who set up forwarding.
Common causes
- Two mailboxes that forward to each other.
- MX records or relay rules that send the domain's mail back to a server that already handled it.
- A domain migration where the old and the new system each think the other is responsible.
How to fix it
- 01Look at the Received headers of the returned message to see which servers repeat.
- 02Mailbox owners: check forwarding rules and remove the one that sends mail back.
- 03Domain admins: check MX records and relay or smart host settings for a circle.
- 04Resend once the loop is fixed; the bounced copy is not retried.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 3463, section 3.5: Network and Routing Statusrfc-editor.org
- RFC 5321, section 6.3: Loop Detectionrfc-editor.org
- Microsoft Learn: Fix NDR error 5.4.6 or 5.4.14 in Exchange Onlinelearn.microsoft.com
- IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org
Related codes
- 5.2.4Mailing list could not be expanded.The address is a mailing list, and the server could not turn it into the list of member addresses, so the message did not go out to the list.
- 5.3.5Receiving system misconfigured.The receiving system's own configuration does not allow it to accept this message, a problem only its administrator can fix.
5.4.6 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce