Outlook 2010/2013 Post Migration Process
When a mailbox move is in process, users will not see any interruption in Microsoft Outlook. Once the move reaches the completing phase, the server will lock the mailbox and Outlook will disconnect. You may see this at the bottom of the Outlook window:

This message will alternate between Trying to connect and Disconnected. This period may last up to 10 minutes. After this time, you will receive the following notification:

At this point, click OK and close and restart Outlook. It is possible that a credential prompt may be received simultaneously with this message. Click Cancel on the prompt, then close and restart Outlook. Make sure to give sufficient time for it to fully exit before re-opening the application.
While the client was disconnected, any messages that were sent prior to disconnection will be held in the Outbox until the connection to the server is restored. Upon exiting, Outlook may warn that some messages have not been sent.

Be sure to click Exit Without Sending. The messages will be sent on next login.
Upon re-login, a credential prompt will be displayed. Authenticate using email address and password. No other credentials will be accepted in this box. Be sure to check the Remember my credentials checkbox so it does not prompt again.

Once logged in, you may see the notification to close and re-open Outlook again. Make sure to do this if prompted, as Outlook may require additional automatic reconfiguration. If prompted by autodiscover to allow changes during this period, make sure to check the Don’t ask me about this website again check box and choose Allow.