During the Migration you get an error in the Migration Console as follows:
"The Admin/Migration user must have 'Receive As' permission
to the mail store.
This permission must be assigned using the Exchange
Management Shell (PowerShell) as specified in the
documentation."
Also in the log you might see the following error:
14:54:19 ERROR: [4970-24-52-8004011D]Unable to open mailbox for user 'XXXX@Company.com'
14:54:19 OpenMsgStore:
14:54:19 MAPI error 8004011D
14:54:19 Error: The attempt to log on to Microsoft Exchange has failed.