Microsoft 365 5.4.6 / 5.4.14
Routing loop detected
Microsoft 365 5.4.6 / 5.4.14 means: routing loop detected. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Routing loop detected
Possible cause
A configuration error caused an email loop. On-premises Exchange server generates error 5.4.6 (you see this code in hybrid environments). Exchange Online generates error 5.4.14. By default, after 20 iterations of an email loop, Exchange interrupts the loop and generates an NDR to the sender of the message.
Provider's advice
This error occurs if the delivery of a message generates another message in response. That message then generates a third message, and the process is repeated, creating a loop. To help protect against exhausting system resources, Exchange interrupts the mail loop after 20 iterations. Mail loops are typically created because of a configuration error on the sending mail server, the receiving mail server, or both. To determine whether automatic message forwarding is enabled, check the sender's and recipient's mailbox rules configuration.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.4.14: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.4.0 other or undefined network or routing status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
Microsoft says Exchange Online stops after 20 iterations of a loop by default and returns an NDR. Loops are usually caused by hybrid setups where an on-premises server and Exchange Online each consider the other authoritative for a domain, or by mail flow rules and forwarding that send messages back to the origin. Check accepted domain settings, connectors and MX records on both sides.
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 5.4.6 / 5.4.14?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Microsoft 365 5.4.6 / 5.4.14 mean?
- Microsoft 365 5.4.6 / 5.4.14 means: routing loop detected. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 5.4.6 / 5.4.14?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Would verifying my list have prevented Microsoft 365 5.4.6 / 5.4.14?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05