"552 5.3.4 Header size exceeds fixed maximum size" from Exchange

Emails with very long header sections (typically from mailing lists, forwarding chains, or automated systems) are rejected by Exchange because they exceed the default header size limit.

Fix

Increase the maximum header size in Exchange Management Shell:

set-receiveconnector -identity Default* -MaxHeaderSize 256kb

Then move any affected emails from the BADMAIL folder to PICKUP to retry delivery.