How to Fix Missing homeMDB Attribute in Exchange Arbitration Mailboxes

Learn how to fix the issue of missing homeMDB attribute in Exchange arbitration mailboxes, which can cause problems during mailbox migration.

Arbitration mailboxes are special system mailboxes that store various types of system data and manage messaging approval workflow in Exchange Server. Sometimes, these mailboxes may have missing or invalid homeMDB attributes, which can prevent them from being moved to another database or server. In this article, we will explain what the homeMDB attribute is, why it is important, and how to fix it if it is missing or invalid.

What is the homeMDB Attribute?

The homeMDB attribute is an Active Directory property that specifies the distinguished name of the mailbox database that hosts the mailbox. For example, the homeMDB attribute of a mailbox in the database DB01 on the server EX01 would look something like this:

CN=DB01,CN=Databases,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=First Organization,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=contoso,DC=com

The homeMDB attribute is used by Exchange to locate the mailbox and perform various operations on it, such as moving, deleting, or updating it. Therefore, it is essential that the homeMDB attribute is set correctly and matches the actual location of the mailbox in the Exchange organization.

Why is the homeMDB Attribute Missing or Invalid?

There are several possible reasons why the homeMDB attribute of an arbitration mailbox may be missing or invalid, such as:

• The arbitration mailbox was deleted and recreated, but the homeMDB attribute was not set properly.
• The mailbox database that hosts the arbitration mailbox was moved, renamed, or deleted, but the homeMDB attribute was not updated accordingly.
• The arbitration mailbox was corrupted or damaged, and the homeMDB attribute was lost or changed.
• The permissions on the arbitration mailbox or the mailbox database were modified, and the homeMDB attribute became unwritable or inaccessible.

How to Fix the Missing or Invalid homeMDB Attribute?

To fix the issue of missing or invalid homeMDB attribute in Exchange arbitration mailboxes, you need to perform the following steps:

1. Check the current value of the homeMDB attribute for the arbitration mailbox using the Active Directory Users and Computers (ADUC) tool or the Exchange Management Shell (EMS).
2. If the homeMDB attribute is missing, you need to recreate the arbitration mailbox using the Exchange setup.exe command with the /PrepareAD switch.
3. If the homeMDB attribute is invalid, you need to set it to the correct value using the Set-Mailbox cmdlet with the -Arbitration and -Database parameters.
4. Verify that the homeMDB attribute is fixed and the arbitration mailbox is working properly.

Step 1: Check the Current Value of the homeMDB Attribute

To check the current value of the homeMDB attribute for the arbitration mailbox, you can use either the ADUC tool or the EMS.

Using the ADUC Tool

1. Open the ADUC tool and enable the Advanced Features option from the View menu.
2. Locate the arbitration mailbox under the Microsoft Exchange System Objects container and open its properties.
3. Click on the Attribute Editor tab and scroll down to find the homeMDB attribute.
4. Check the value of the homeMDB attribute and note it down.

Using the EMS

1. Open the EMS and run the following command to get the value of the homeMDB attribute for the arbitration mailbox:

   Get-Mailbox -Arbitration | Select Name,homeMDB

2. Check the output and note down the value of the homeMDB attribute.

Step 2: Recreate the Arbitration Mailbox

If the homeMDB attribute is missing for the arbitration mailbox, you need to recreate the arbitration mailbox using the Exchange setup.exe command with the /PrepareAD switch. This command will create all the required system mailboxes, including the arbitration mailboxes, and set their homeMDB attributes correctly.

To recreate the arbitration mailbox, follow these steps:

1. Mount the Exchange installation media or ISO file and note down the drive letter.
2. Open an elevated command prompt and navigate to the drive letter of the Exchange installation media.
3. Run the following command to recreate the arbitration mailbox:

   Setup.exe /IAcceptExchangeServerLicenseTerms /PrepareAD

4. Wait for the command to complete and check the output for any errors.

Step 3: Set the homeMDB Attribute to the Correct Value

If the homeMDB attribute is invalid for the arbitration mailbox, you need to set it to the correct value using the Set-Mailbox cmdlet with the -Arbitration and -Database parameters. This command will update the homeMDB attribute to match the actual location of the arbitration mailbox in the Exchange organization.

To set the homeMDB attribute to the correct value, follow these steps:

1. Open the EMS and run the following command to get the name of the mailbox database that hosts the arbitration mailbox:

   Get-Mailbox -Arbitration | Select Name,Database

2. Note down the name of the mailbox database and run the following command to set the homeMDB attribute to the correct value:

   Set-Mailbox -Arbitration -Identity "<Arbitration Mailbox Name>" -Database "<Mailbox Database Name>"

3. Replace the placeholders with the actual values and run the command for each arbitration mailbox that has an invalid homeMDB attribute.

Step 4: Verify that the homeMDB Attribute is Fixed

To verify that the homeMDB attribute is fixed and the arbitration mailbox is working properly, you can use the ADUC tool or the EMS to check the value of the homeMDB attribute again. You can also use the Test-MapiConnectivity cmdlet to test the connectivity and functionality of the arbitration mailbox.

To verify that the homeMDB attribute is fixed, follow these steps:

1. Open the ADUC tool or the EMS and check the value of the homeMDB attribute for the arbitration mailbox. It should match the distinguished name of the mailbox database that hosts the arbitration mailbox.
2. Open the EMS and run the following command to test the connectivity and functionality of the arbitration mailbox:

   Test-MapiConnectivity -Identity "<Arbitration Mailbox Name>"

3. Check the output and make sure that the result is Success.

Frequently Asked Questions (FAQs)

Question: What are arbitration mailboxes?

Answer: Arbitration mailboxes are special system mailboxes that store various types of system data and manage messaging approval workflow in Exchange Server. They are used for features such as federation, migration, moderation, e-discovery, and OAB generation.

Question: How many arbitration mailboxes are there in Exchange Server?

Answer: The number of arbitration mailboxes depends on the version of Exchange Server. Exchange 2010 has three arbitration mailboxes, Exchange 2013 has five arbitration mailboxes, and Exchange 2016 and 2019 have seven arbitration mailboxes.

Question: How to find arbitration mailboxes in Exchange Server?

Answer: You can use the Get-Mailbox cmdlet with the -Arbitration parameter to find arbitration mailboxes in Exchange Server. For example, the following command will list all the arbitration mailboxes in the Exchange organization:

Get-Mailbox -Arbitration

Question: How to move arbitration mailboxes in Exchange Server?

Answer: You can use the New-MoveRequest cmdlet with the -Arbitration parameter to move arbitration mailboxes in Exchange Server. For example, the following command will move an arbitration mailbox to another mailbox database:

New-MoveRequest -Arbitration -Identity "<Arbitration Mailbox Name>" -TargetDatabase "<Mailbox Database Name>"

Summary

In this article, we have explained how to fix the issue of missing or invalid homeMDB attribute in Exchange arbitration mailboxes, which can cause problems during mailbox migration. We have shown how to check, recreate, and set the homeMDB attribute using the ADUC tool and the EMS. We have also answered some frequently asked questions about arbitration mailboxes.

Disclaimer: The information in this article is provided as-is and does not guarantee any results. You are responsible for your own actions and should always test the commands and procedures in a lab environment before applying them to a production environment. We are not liable for any damages or losses caused by following the instructions in this article.