Generic SMTP routing loop

Fix SMTP 5.4.6 routing loop detected

5.4.6 means a configuration caused the message to repeat a delivery route. Find the repeated hosts or address transformations in Received headers and message trace.

Problem description

A generic DSN may report:

550 5.4.6 Routing loop detected

Diagnostic availability: public DNS cannot expose private connectors, transport rules, aliases, or hybrid topology. This page intentionally has no checker CTA.

Is this temporary or permanent?

The loop permanently failed this message. Retrying without changing the route repeats the failure.

Typical loop patterns

Two gateways each treat the other as the next hop.
A forwarding address returns mail to its source domain.
Hybrid cloud and on-premises accepted-domain settings disagree.
Old and new providers remain active during a migration.

How to fix it

Read Received headers from bottom to top and mark repeated hosts.
Use message trace on every managed platform.
Map MX, smart hosts, connectors, aliases, and transport rules.
Choose one authoritative destination per recipient namespace.
Change the narrow route and test all affected address classes.

Frequently asked questions

What does SMTP 5.4.6 mean?

The message repeatedly traversed the same routing path until the mail system detected a loop.

Can DNS alone identify the loop?

Usually not. Received headers, message trace, connectors, aliases, and private routes are required.

How is Microsoft 5.4.14 related?

Microsoft documents 5.4.14 for Exchange Online routing loops and 5.4.6 for the related on-premises or hybrid condition.

Related email diagnostics

Primary sources: IANA SMTP Enhanced Status Codes registry and RFC 3463.