User can’t access on-prem mailbox after migration to Exchange Online

After migrating a user mailbox to Exchange Online (from Exchange 2016) the user could no longer access some shared mailboxes that are still hosted on-premise in Exchange 2016 (permissions are fine, they could access them prior to being migrated).  When trying to add them via Account Settings in Outlook the following error is displayed:

The name cannot be resolved.  The name cannot be matched to a name in the address list.

It’s worth noting that these ‘shared’ mailboxes are not ‘Shared Mailboxes’ in the true sense, they are simply user mailboxes that the other users have been granted access to.  So, to fix the issue, I had to do two things:

  • Change the UPN in Active Directory from to
  • Move the user account in Active Directory to an OU that is sync’d to Office 365

One these had been done I ran a sync for AADC (Start-ADSyncSyncCycle -PolicyType Delta) and after a short while the mailboxes appeared automatically in Outlook for the users.

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