Sometimes during migration of Mailboxes – especially in cross-forest migrations, some of mailboxes may become corrupted. Mailboxes can fail test Test-MapiConnectiviy, user maybe unable to login into mailbox using OWA or messages just not coming in and not going out.
General solution here is disconnect and then connect mailbox again. You only should keep in mind that when you disconnecting Mailbox all Email addresses will be deleted so there is need to note all of them before disconnection.
Following script will do the work: