Cannot open mailbox /cn=Microsoft System Attendant. (4377288)

During an Exchange Hybrid project, mail migration tasks may encounter errors when running that generate messages indicating the inability to open the "Microsoft System Attendant" mailbox. The error messages are typically logged as follows: 

07/08/2024 06:54 Error Hybrid Exchange: PowerShell (Auto Reply): Cannot open mailbox /o=Domain/ou=Exchange Administrative Group (FWERKJHETR)/cn=Configuration/cn=Servers/cn=EXCH01/cn=Microsoft System Attendant.

The errors suggest that the system is trying to interact with a "Microsoft System Attendant" mailbox, which cannot be opened due to potential issues with the associated database. Although the "System Attendant" mailbox is no longer present in newer versions of Exchange, legacy references can still exist, and issues might arise if the database hosting the required arbitration mailbox is unmounted or unavailable.

This problem may also be triggered by temporary unavailability due to maintenance, upgrades, or other actions that affect the Exchange server's configuration.

• Check Exchange Database Status:

  • Verify if the database hosting the arbitration mailbox is mounted. If the database is dismounted, remount it using Exchange Management Shell (EMS):
     

    Mount-Database -Identity "<DatabaseName>"

• Recreate Missing Arbitration Mailbox:

  • If the arbitration mailbox is missing, it can be recreated by preparing the Active Directory (AD) and enabling the mailbox:
     

    Setup.exe /PrepareAD Enable-Mailbox -Arbitration -Identity "Migration.8f3e7716-2011-43e4-96b1-aba62d229136"

• Review Maintenance Logs:

  • Since the errors occurred around the same time on multiple occasions, review the server logs to check if any maintenance or upgrade tasks were performed that might have affected database availability. Address any identified issues and ensure that critical mailboxes are not impacted during such activities.