Answer
Email has just arrived for another destination so GMS has created a queue for the remote server. If any more email arrives for this destination it will be added to the queue so that all email messages are delivered during the same session to the remote server. When queues reach over a certain size, additional queues are created so that multiple sessions will start to send email to the remote server.
In this case, session Undefined created a queue called Undefined.
This is the expected result, no action is required by you.
See Also:
Keywords:create post queue 8381