Answer
A message was sent to be delivered to multiple accounts on GMS but it could not be delivered to one of them. So GMS has sent a DSN error message back (as defined by Internet Standard RFC1894) to the sender of the message indicating that message was not delivered.
In this case, session Undefined was processing a message from a server at Undefined. The message was apparently from Undefined to Undefined and the DSN error message was sent to Undefined.
This is the correct result, no action is required by you.
See Also:
Keywords:DSN delivery failure RFC1894 8287