554 5.4.6 Message exceeded 50 hops, this may indicate an email loop
Sample bounce line
554-5.4.6 Message exceeded 50 hops, this may indicate an email loop. 554 5.4.6 For more information, go to https://support.google.com/mail/answer/81126
What it means
The message collected more than fifty Received headers before reaching Gmail, which almost always means two systems are handing the same message back and forth. Gmail stopped it rather than let it circulate.
Why it happens
- Two mailboxes forwarding to each other, directly or through a third address.
- A forwarding rule that points back at an alias which resolves to the original mailbox.
- An auto-reply or vacation rule answering another auto-reply.
- A split mail flow where an on-premises server and a cloud tenant each believe the other is authoritative for the domain.
How to fix it
- Read the Received headers of a looping message from the bottom up: the two hostnames that keep repeating are the loop.
- Remove one side of the forwarding pair, then send one test message.
- Check aliases and distribution groups as well as mailbox rules - a group that contains its own address loops just as well.
- Where mail is split between on-premises and cloud, make one of them authoritative and point the other at it.
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