ExternalEmailAddress is mandatory on MailUser.

Hi Michael W. Crocker

Welcome to Microsoft Q&A and thank you for reaching out to us.

Based on your descriptions, I understand that you're encountering the error "ExternalEmailAddress is mandatory on MailUser" when running the Get-Mailbox -Arbitration command, and that mailboxes are being incorrectly identified as MailUsers. This issue may be related to an incomplete or interrupted installation of the latest Cumulative Update (CU) for Exchange Server 2019, which can affect how mailbox objects are classified and accessed.

To help me better understand the issue more accurately, could you please share some information (a screenshot of the exact error message you receive when sending email. Kindly blur or hide any sensitive information in the image) as below:

• Full error output when running the command:
• Exact error message or screenshot shown in the Exchange Admin Center (EAC) when attempting to open or modify a user.

Get-Mailbox -Arbitration | Format-List Name,RecipientTypeDetails,ExternalEmailAddress

• Additionally, may I know:
  • The CU installation completed successfully or has issue midway.
  • You ran the following commands during the CU installation:
    • Setup.exe /PrepareSchema /IAcceptExchangeServerLicenseTerms
    • Setup.exe /PrepareAD /IAcceptExchangeServerLicenseTerms
• Whether all mailboxes are affected or only specific ones (e.g., arbitration mailboxes, user mailboxes, shared mailboxes).
• Any recent changes to Active Directory or Exchange services prior to the issue.

Please let us know if you have any questions or need assistance collecting the above details.

I look forward to your response.

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Hi Michael W. Crocker

I hope you are doing well and thank you for your update.

Based on your response, the installation of Cumulative Update 15 (CU15) did not complete successfully, which caused Exchange to misread mailbox attributes and return the error ExternalEmailAddress is mandatory on MailUser.

At this situation, the effective and supported recommendation from my findings is restoring functionality is to perform a Recovery Setup of Exchange 2019 using CU15, then apply the latest Security Update, and finally remount or restore your mailbox databases. This approach reinstalls the Exchange application using the configuration already stored in Active Directory, without affecting your existing mailbox data.

For reference and detailed information, the full Microsoft guidance for recovery setup is available here: Recover Exchange server, recover lost Exchange Server, Lost Exchange Server recovery | Microsoft Learn

(Note: The /RecoverServer command must use the same CU version as the Exchange build that was previously installed. If you had partially installed CU15 before the failure, use CU15 again.)

You can find the official list of updates here to install the latest Security Update for Exchange 2019 CU15 from Microsoft: Upgrade Exchange to the latest Cumulative Update | Microsoft Learn

Once the recovery setup and patching are complete:

• If databases are healthy, run:

    Mount-Database -Identity "Mailbox Database Name"
  

• If restoring from backup, place the .edb and log files in their original locations before mounting.

For your detailed information:

Mount-Database (ExchangePowerShell) | Microsoft Learn

Restore data using a recovery database | Microsoft Learn

Additionally, this summary is based on my findings from the community and several relevant threads. However, it may not accurately reflect the behavior in question. To help you reach your goal more effectively, I recommend posting a thread on the Microsoft Tech Community forum if issue still persists. It’s a great platform for deeper technical discussions and connecting with individuals who have hands-on experience and expertise. They’re best positioned to provide guidance and valuable insights on this topic.   

Apologies for redirecting you to the related development team support as the moderators in this community have limited resources to check the backend information, and to get the fast and better assistance.

And if the information I provided has been even partially helpful, please consider marking this answer as accepted so it can serve as a helpful reference for others facing similar challenges. Your feedback also contributes to building a stronger and more supportive community.

Thank you for your understanding.