How To Fix Error – Exchange Database Is Mandatory On Usermailbox

When expanding the setup in your existing Exchange environment due to load, you would add a new Exchange Server and split the mailboxes between the two servers or set up a Database Availability Group (DAG). Sometimes, the installer fails while introducing a new Exchange Server in the setup and throws the “Database is mandatory on UserMailbox” error. Due to this, you will not be able to continue with the setup.

You can see more logs in the Exchange Setup Logs. These are usually found in the default folder: C:\ExchangeSetupLogs\ExchangeSetup.txt. This is a good place to get more information about the problem. You may see the below error message.

[01/01/2023] [1] [ERROR] Database is mandatory on UserMailbox.

[01/01/2023] [1] [ERROR-REFERENCE] Id=SystemAttendantDependent___03c2eeded42c32b6bf14ee3fe543df60 Component=EXCHANGE15:\Current\Release\PIM Storage\Discovery

As you can see, the error is coming from the System Attendant mailbox, which is a system mailbox managed by the Exchange Server.

To check if there are any issues with the system mailboxes, you can use the Get-Mailbox command on the existing Exchange Server (as given below).

Get-Mailbox -Arbitration | Select Name | Format-Table

In a normal scenario, you would not encounter any error. However, in this case, you will get the following message under the System Mailbox with the referred GUID in the installer.

WARNING: The object SKARO.LOCAL/Users/SystemMailbox{03c2eeded42c32b6bf14ee3fe543df60} has been corrupted, and it's in an inconsistent state. The following validation errors happened:

WARNING: Database is mandatory on UserMailbox.

Why This Error Occurs?

This error may occur due to many reasons, such as:

• Different versions of Exchange Server.
• Different setups and operating systems.
• Different configurations.

However, the most common one is that the HomeMDB parameter is missing from the mailbox. This parameter would be missing due to a third-party application, misconfiguration, or human error. This parameter cannot be found in the Exchange Admin Center (EAC). This property can be found in the Active Directory. To verify this, follow the steps below:

• Open the Active Directory Users and Computers.
• Click on View and then click on Advanced Features.
• In the default Users OU, you will find the system mailbox.
• Right-click on the user and click on Properties.
• Click on the Attribute Editor tab.
• Find the homeMDB parameter.

How to Resolve the “Exchange database is mandatory on UserMailbox” Error?

As you can see, the error usually occurs when the home mailbox database is missing for the specified system mailbox. To resolve the issue, follow these instructions:

• One of the solutions is to recreate the system mailbox. Before starting this, you need to confirm and accept that once you run the commands, it will not restore anything and it would re-create the system mailbox.
• If you have a backup of the mailbox, you can restore the mailbox from backup to a different mailbox database. It would be preferred over the above solution.
• For re-homing a system mailbox, you can use the Set-Mailbox command (as given below) to repair a broken mailbox and assign it to another mailbox database, along with the arbitration parameter to instruct the command that you are working with a system mailbox.

Set-Mailbox -Identity "SystemMailbox{ <guid> }" -Database "<database name>" -Arbitration

• After executing the above command, you need to confirm twice. If you have more mailboxes, this needs to be repeated for each and every database. Once done, you can confirm if the issue has been fixed by running the below command.

Get-Mailbox -Arbitration | Select Name | Format-Table

If the above does not result in any error, you can go ahead and re-try the installation.

When reinstalling the Exchange server on Active Directory (AD) where a previous Exchange installation existed, this error may arise. To resolve this, navigate to the ‘Users’ section of ‘AD Users and Computers’ and delete any old records (users) with names similar to ‘SystemMailbox{c9a42345-65d2-4299-b3d0-85c1f6a29c6d}’ that remain from the previous Exchange installation. Now check if issue is resolved.

Conclusion

If you are still facing the “Database is mandatory on UserMailbox” error, it means that the main database has been corrupted or the Exchange Server has an issue after a migration or Cumulative Update (CU). In such cases, it’s difficult to resolve the matter in a timely fashion, without affecting the users or keeping data loss to a minimum.

However, you can use a third-party application, like Stellar Repair for Exchange that can reduce the time of recovery. With Stellar Repair for Exchange, you can open multiple EDB files in any condition and browse through them. You can granularly export the recovered database to PST and other file formats. You can also export the recovered database directly to a new Exchange mailbox database with automatic mailbox mapping.

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About The Author

Ravi Singh

Ravi Singh is a Senior Writer at Stellar®. He is an expert Tech Explainer, IoT enthusiast, and a passionate nerd with over 7 years of experience in technical writing. He writes about Microsoft Exchange, Microsoft 365, Email Migration, Linux, Windows, Mac, DIY Tech, and Smart Home. Ravi spends most of his weekends working with IoT (DIY Smart Home) devices and playing Overwatch. He is also a solo traveler who loves hiking and exploring new trails.