Answer
GMS has identified a message that has bounced from a remote system and been returned to the local "NULL" account indicated by an address of <>. Therefore GMS can not deliver the email as it contains no local account information.
In this case session Undefined was processing a message from a server at Undefined apparently from Undefined to Undefined.
If you wish to check the message that was bouncing, switch on the option in the configuration interface and check the /BadMes directory for the domain for Undefined.
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Keywords:Bounce bouncing mail 8226