I ran into a strange issue today after moving a mailbox from Exchange 2010 to 2016. The move was successful and the move request has been completed/removed. Outlook is connecting to Exchange 2016 as expected. However, when this user logs in through OWA, they are still getting redirected back to Exchange 2010. Everything is functional in OWA on 2010, they just aren't getting the new visual OWA 2016 experience. I tried cycling the OWA app pool on each server, but it didn't seem to help. Other mailboxes moved in the same batch did not experience this problem.