{"id":76870,"date":"2021-03-25T06:54:37","date_gmt":"2021-03-25T06:54:37","guid":{"rendered":"https:\/\/www.stellarinfo.com\/blog\/?p=76870"},"modified":"2025-12-24T09:04:03","modified_gmt":"2025-12-24T09:04:03","slug":"cannot-open-mailbox-first-organization-exchange-administrative-group","status":"publish","type":"post","link":"https:\/\/www.stellarinfo.com\/blog\/cannot-open-mailbox-first-organization-exchange-administrative-group\/","title":{"rendered":"How to Fix Error \u201cCannot Open Mailbox \/o=First Organization \/ou=Exchange Administrative Group\u201d?"},"content":{"rendered":"

Exchange Server offers various functionalities and features for collaboration and for smooth functioning of messaging applications, such as Outlook. One of the features in Exchange Server is the Add-ins feature that allows to create, control, and manage add-ins for Outlook. These add-ins extend the functionalities of the Outlook client by adding information and tools.<\/p>

The users can install the add-ins for Outlook by default. However, as an admin, you would not want the users to install any incorrect third-party add-ins, which can hinder the communication or operability of Outlook. To add, manage, and control add-ins, you can open the Exchange Admin Center (EAC), click Organization, and then click on Add-ins. However, sometimes, when accessing the Add-ins tab, you can encounter an error message, similar to the below:<\/p>

“Cannot open mailbox \/o=My Organization\/ou=Exchange Administrative Group (GUI)\/cn=Configuration\/cn=Servers\/cn=EX01\/cn=Microsoft System Attendant.”<\/strong><\/p>

\"However,<\/figure>

This error usually occurs if any Exchange Services are stopped or any database is dismounted from the server. Below, we will see how to fix this error.<\/p>

Methods to Fix Cannot Open Mailbox \/o=First Organization \/ou=Exchange Administrative Group Error<\/strong><\/h2>

Depending on the cause of error, you can follow the below-given methods to fix this error in Exchange Server.<\/p>

Method 1: Check and Start Exchange Services
<\/strong><\/h3>

For proper functioning of the Exchange Server, all the services must be in the Running state. If any of the Exchange services are stopped, then you may face issues and errors when accessing certain features. So, check and ensure that all the Exchange services are running. To do so, follow these steps:<\/p>

    \n
  1. Open the Server Manager<\/a>. Click on Tools<\/strong> and select Services<\/strong> from the list.<\/li>\n<\/ol>
    \"Click<\/figure>

    2. On the Services<\/strong> screen, check that all services of the Exchange Server are running. Make sure that any service, which has the Startup Type<\/strong> set to Automatic<\/strong>, is started.

    3. If the status of any service(s) is Stopped<\/strong>, right-click on it, and select Start<\/strong>.<\/p>

    \"If<\/figure>

    Alternatively, you can use the following PowerShell cmdlet to restart all the Exchange services:<\/p>

    Get-Service *Exchange* | Where {$_.DisplayName -notlike “*Hyper-V*”} | Restart-Service –Force<\/strong><\/p>

    \"Alternatively,<\/figure>

    The error message reads as:<\/strong><\/p>

    “Cannot open mailbox \/o=My Organization\/ou=Exchange Administrative Group (GUI)\/cn=Configuration\/cn=Servers\/cn=EX01\/cn=Microsoft System Attendant.”<\/em><\/p>

    Once all the services are started, try to access the Add-ins tab in EAC. If the error persists, follow the next method.<\/p>

    Method 2: Check if all the Databases are Mounted<\/strong><\/h3>

    You may also encounter the error Cannot open mailbox \/o=First Organization \/ou=Exchange Administrative Group,<\/strong> if one or more databases are not mounted. So, make sure that the status of all databases is set to ‘Mounted.’ To check and mount the databases using the Exchange Admin Center (EAC), follow these steps:<\/p>