While attempting to migrate a mailbox in Exchange, you may encounter the following error:

"The mailbox database is temporarily unavailable."

Cause

This error typically occurs when the mailbox database that hosts the user's mailbox is dismounted. Migration cannot proceed if the database is not mounted and accessible.

Resolution

To resolve this issue, mount the dismounted mailbox database from the Exchange Admin Center (EAC).

Steps to Resolve

  1. Log in to the Exchange Admin Center (EAC).


     
  2. In the left navigation pane, go to Servers.
  3. Click on Databases.


     
  4. Select the affected mailbox database.
  5. Click on the three dots (…), then choose Mount.


     
  6. The database will be mounted.


     

PowerShell Alternative

If you prefer using the Exchange Management Shell instead of the Exchange Admin Center (EAC), you can mount the mailbox database using the following command:

Mount-Database -Identity "MailboxDatabase01"

If the database is successfully mounted, the command will complete without errors. After mounting, you can retry the mailbox migration process.

Conclusion

Once the mailbox database is successfully mounted, the error "The mailbox database is temporarily unavailable" should no longer appear. You can then resume or restart the mailbox migration process without any further interruption.