SMTP routing loop
SMTP 5.4.6: Routing loop detected
The message returned to the same route too many times. Public DNS may be involved, but private connectors, aliases, and hybrid routing are common causes.
The registered response text is:
5.4.6 Routing loop detected
A 5.4.6 status rejects this delivery. Correct the loop before submitting a new message; some systems use 4.4.6 while retrying the same condition.
Find the loop
- Save the full bounce and original message headers.
- Read the Received headers from bottom to top and mark repeated hosts.
- Check message trace on each managed mail platform.
- Map the connectors, smart hosts, aliases, forwarding rules, and accepted domains between those hosts.
- Choose one authoritative destination for the affected recipient domain.
- Correct the narrow route, then test internal and external recipients.
Do not change SPF or add more retries. Neither can stop a message from being routed back to the same system.
Frequently asked questions
Can public DNS prove where the routing loop is?
Usually not. You need the full Received chain, message trace, and the private connector or forwarding configuration of each repeated system.
Is SMTP 5.4.6 safe to retry?
Do not resubmit it unchanged. The route must be corrected first. Some systems report the routing-loop condition as temporary 4.4.6, but repeated retries still cannot fix the configuration.
Related email diagnostics
Primary sources: IANA SMTP Enhanced Status Codes, RFC 3463: Enhanced Mail System Status Codes.