SMTP enhanced status code 4.4.6

What the registry says

In practice

Loops happen when two systems forward to each other, for example a mailbox that forwards to an alias that points back, or two servers that each think the other is the final destination. Microsoft reports 5.4.6 from on-premises Exchange and 5.4.14 from Exchange Online, and Gmail stops after 50 hops with 554 5.4.6. The loop has to be found in forwarding rules or connector settings.

What the sender should do

Check forwarding rules and connectors on both sides. A recipient forwarding to an address that forwards back is a common cause.

How providers use X.4.6

Quoted from each provider’s own error reference.

• Gmail 554 5.4.6

  Message exceeded 50 hops, this may show a mail loop. For details, go to SMTP relay service error messages.

Common questions

What does 4.4.6 mean?

X.4.6 means a routing loop: the message was forwarded too many times between servers, because of bad routing or a forwarding rule that points back on itself.

What is the difference between 4.4.6 and 5.4.6?

They share the subject and detail digits, so they report the same condition, "Routing loop detected". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.

What should I do about 4.4.6?

Check forwarding rules and connectors on both sides. A recipient forwarding to an address that forwards back is a common cause.

Would an email verifier have caught 4.4.6?

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.