Migration to an on premise Exchange 2016 server fails immediately with "ERROR: [4965-24-26-80040111]Unable to open target mail store"
Other errors that can be present in the logs are "ERROR: [8803-94-91-00040203]Unable to open server 'Exchange FQDN'"
Migration admin account is assigned Full Access to all mailboxes. Receive-as permissions is all that we require.
Over provisioning of the Migration admin account caused too many automapped mailboxes to open in the Outlook profile. This lead to an inability to open a mailbox of a user that is being migrated.