Microsoft mail-routing error
Fix Outlook 550 5.4.14 routing loop detected
5.4.14 means Exchange Online detected a routing loop. Trace repeated systems in message trace and Received headers; public MX alone does not reveal private connectors or hybrid routes.
The NDR reports:
550 5.4.14 Routing loop detected
The message failed permanently after looping. Correct the route and send a new message; retrying the same topology repeats the loop.
Typical causes
- Microsoft 365 and an on-premises server each route the domain to the other.
- A smart host sends mail back to the connector that originated it.
- Accepted-domain type or centralized mail transport is inconsistent.
- A migration leaves overlapping old and new routes.
How to resolve it
- Use message trace and Received headers to find the repeating pair.
- Map MX, connectors, smart hosts, and accepted domains.
- Choose one authoritative next hop for each recipient namespace.
- Correct the narrow connector or accepted-domain setting.
- Test internal, external, and hybrid recipients separately.
Frequently asked questions
What is a mail routing loop?
Two or more systems repeatedly route the same message back to one another until a hop or loop limit is reached.
Is 5.4.14 the Exchange Online code?
Microsoft says Exchange Online generates 5.4.14, while on-premises Exchange can generate 5.4.6 for the related loop condition.
Can an MX check locate the loop?
No. Message trace, connector configuration, and Received headers are needed to identify the repeating private route.
Related email diagnostics
Primary sources: Microsoft Exchange Online NDR reference.