Microsoft mail servers 550 5.4.14
Routing
The receiving service refused the route, connector, or relay path. The recipient may still be valid, so fix routing before making a suppression decision.
Decision
Do not retry unchanged. Fix the sender setup first.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- Any SMTP stage
- Endpoint
- Inbound MX
Routing loop detectedStart here
- 1
Check MX records, forwarding paths, and connector configuration for loops or relay denial.
- 2
Retry only after the routing path has been corrected.
Confirm the cause
- Match provider, SMTP stage, endpoint, reply code, enhanced code, and diagnostic text.
- Run the sender, recipient, DNS, and content checks that match the diagnostic wording.
- Test with one recipient before retrying bulk traffic.
App handling
- Treat this as a sender setup incident, not a bad recipient.
- Hold similar mail for the same sender domain or provider until the fix is verified.
- Retry after a fresh single-recipient test succeeds.
Avoid
- Do not retry unchanged in a loop.
- Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
- Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
Common causes
- Forwarding, MX, or connector rules send the message back into the same route.
- Two systems are configured as each other's next hop.
- A migration or hybrid mail flow rule still points at an old destination.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampsource IPconnectorTLS result