Merge pull request #10690 from MicrosoftDocs/main

Auto Publish – main to live - 2026-02-09 22:00 UTC

Author: learn-build-service-prod[bot]

Exchange/ExchangeServer/servertoc/toc.yml

@@ -492,6 +492,8 @@ items:
           href: ../setup/event-id-6010-after-cumulative-update-23.md
         - name: Exchange Server 2019 setup does not run correctly using PowerShell
           href: ../setup/ex2019-setup-does-not-run-correctly-started-powershell.md
+        - name: Exchange Server Setup fails and returns “Database is mandatory on UserMailbox” error 
+          href: ../setup/database-mandatory-on-usermailbox.md
         - name: Instal PST Capture tool fails
           href: ../setup/pst-capture-setup-wizard-ended-prematurely.md
         - name: Install Exchange Server 2010 SP2 fails

Exchange/ExchangeServer/setup/database-mandatory-on-usermailbox.md

@@ -0,0 +1,80 @@
+---
+title: Exchange Server Setup fails and returns "Database is mandatory on UserMailbox" error
+description: Explains how to identify and correct a blank homeMDB attribute on essential mailboxes to ensure that Exchange Server Setup finishes successfully.
+author: cloud-writer
+ms.author: meerak
+manager: dcscontentpm
+ms.reviewer: batre, arindamt, v-kccross
+audience: ITPro
+ms.topic: troubleshooting
+ms.custom: 
+  - sap:Setup Issues
+  - Exchange Server
+  - CSSTroubleshoot
+  - CI 6980
+appliesto: 
+  - Exchange Server SE
+ms.date: 02/09/2026
+---
+
+# "Database is mandatory on UserMailbox" error in Exchange Server Setup
+
+## Summary
+
+The article discusses a Microsoft Exchange Server Setup failure that occurs when essential mailboxes, such as arbitration or discovery mailboxes, have blank `homeMDB` attributes. You can resolve the Exchange Server Setup error by fixing the missing mailbox database assignment.
+
+## Symptoms
+
+When you run Exchange Server Setup, the application fails with an error message that resembles the following message:
+
+ > [ERROR] Database is mandatory on `UserMailbox`
+
+##  Cause
+
+You receive this error message if the `homeMDB` attribute is blank on essential mailboxes such as the arbitration mailbox or the discovery mailbox. This problem can occur, for example, if the mailbox host server is incorrectly deleted.
+
+## Resolution
+
+Use the SetupAssist script to identify affected mailboxes. Then configure the `homeMDB` attribute for each mailbox.
+
+### Identify affected mailboxes
+
+On the server on which Setup failed, download and then run the [SetupAssist](https://microsoft.github.io/CSS-Exchange/Setup/SetupAssist/) script. The script returns information about mailboxes as shown in the following example.
+
+:::image type="content" source="media/database-mandatory-on-usermailbox/setup-assist-result.png" alt-text="Screenshot of SetupAssist script output showing mailboxes with blank homeMDB attributes."
+
+ In the example, the `homeMDB` attribute is blank for `DiscoverySearchMailbox`.
+
+### Fix affected mailboxes
+
+To set the `homeMDB` attribute for each mailbox in the output from the SetupAssist script, complete these steps:
+
+1. [Open the Exchange Management Shell](/powershell/exchange/open-the-exchange-management-shell) or [connect to your Exchange server by using remote PowerShell](/powershell/exchange/connect-to-exchange-servers-using-remote-powershell).
+
+1. Run the `[Get-MailboxDatabase](/powershell/module/exchangepowershell/get-mailboxdatabase)` cmdlet to get a list of available mailbox databases.
+
+   ```powershell
+
+   Get-MailboxDatabase
+
+   ```
+
+1. Run the `[Set-Mailbox](/powershell/module/exchangepowershell/set-mailbox)` cmdlet to set the `homeMDB` attribute one time for each mailbox in the output from the  SetupAssist script. For each confirmation message that displays, select **Yes**. When you run the cmdlet, use a value that uniquely identifies the database, such as the distinguished name (DN), the GUID, or its name.
+
+   ```powershell
+
+   Set-Mailbox “\<*mailbox DN*\>” -Database “\<*database name*\> “
+
+   ```
+
+   Example:
+
+   ```powershell
+
+   Set-Mailbox "CN=DiscoverySearchMailbox {D919BA05-46A6-415f-80AD-7E09334BB852},CN=Users,DC=contoso,DC=lab" -Database "Mailbox Database 1923094126" 
+
+   ```
+
+1. Close the Exchange Management Shell or PowerShell session.
+
+1. Run Exchange Server Setup again.

Exchange/ExchangeServer/setup/media/database-mandatory-on-usermailbox/setup-assist-result.png

@@ -32,7 +32,7 @@ Work together with the resource tenant owners to determine why the service princ
 
 | Scenario | Action |
 |--|--|
-| The service principal is supposed to be disabled. | Don't do anything. Access is intentionally blocked. We don't expect or recommend that resource tenant administrators of first-party applications disable the respective service principal. Microsoft Services automatically provisions and manages the service principals. |
+| The service principal is supposed to be disabled. | Don't do anything. Access is intentionally blocked. We don't expect or recommend that resource tenant admins of first-party applications disable the respective service principal. Microsoft Services automatically provisions and manages the service principals. It's possible that the backing application associated with this service principal was intentionally disabled globally by the home tenant admin, the app owner, or by Microsoft. For more information, see [Deactivate an enterprise application](/entra/identity/enterprise-apps/deactivate-application-portal). |
 | The service principal isn't supposed to be disabled, or it was disabled mistakenly. | Ask the resource tenant owners to re-enable the service principal. One method to re-enable the service principal is to use PowerShell to set the `-AccountEnabled` parameter to `$true`. For more information, see the [Set-AzureADServicePrincipal](/powershell/module/azuread/set-azureadserviceprincipal#example-1-disable-the-account-of-a-service-principal) cmdlet reference. |
 
 ## More information