Sometimes you could see the following Event in the Event Viewer on Exchange 2013, I don’t like seeing these messages on some critical systems.
You would check the MoveRequest and verify if any mailbox move request is stuck or export requests, import requests of migration batches, you should wait, remove, delete it up to you.
You can fix this using the following cmdlet:
Remove-MoveRequest -MoveRequestQueue "DatabaseGuid" -MailboxGuid "RequestGuid"
Afterward, you shouldn’t see that message on event viewer, unless you have another warning about the different mailbox.