503 Service Unavailable: The service that hosts the Exchange server process is not accessible.
Check the event logs on both the Client Access Server (CAS) and Mailbox Server for events that correlate to the errors logged by MigrationWiz.
You may also want to try and restart the Exchange services and/or reboot the server to see if the problem is corrected.
If you are migrating to Office 365 this error indicates Office 365 had a temporary, limited outage. Please wait a couple minutes and resubmit your mailboxes for migration.