Merge pull request #10689 from MicrosoftDocs/AB#6980-kccross-usermailbox

AB#6980 Database is mandatory on Usermailbox - missing homeMDB attribute

Author: Cloud-Writer

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.