Creating an administrator account for Exchange 2016 and 2013 admin login
Existing admin accounts within windows are denied access to mailbox as default so we recommend creating your own specific one for the migration.
On the Exchange 2016 or 2013 server create an account with a mailbox attached.
From the Exchange server do the following:-
Open the Exchange Management Shell.
Enter the following Management Shell command:
Get-Mailbox -ResultSize Unlimited | Add-MailboxPermission -AccessRights FullAccess -User <Enter account name here>
This permission must be re-applied each time a new user with a mailbox is created.