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Many Exchange administrators face this issue when they migrate from a relatively older Exchange version to a modern version of Exchange. For example, from Exchange 2007 to Exchange 2013 or from Exchange 2010 to Exchange 2016. Also, this error is reported when administrators go to the Add-in section in the Exchange admin center (especially in Exchange 2016). Sometimes, Exchange users get this error, and they won’t be able to access their mailboxes in Outlook or Outlook Web Access (OWA).
The error message displayed is:
This error usually happens when the Exchange mailbox database, especially the default mailbox database, is not mounted. The default mailbox database is very important as it holds critical information. So, one has to keep the first mailbox database mounted. Otherwise, this error may occur. Also, this error can occur when some of the required Exchange services are not running.
This error started appearing after some architectural changes in the 2010 version of Exchange e Server. From this version onwards, the System Attendant Mailbox was removed from the first mailbox database that was created after the Exchange installation. But the directory object was not removed from the configuration partition. In addition to this, Microsoft Exchange System Attendant (Mad.exe) was removed from Exchange 2013. This architectural change is one of the reasons for the appearance of the error. It is noted that the incorrect or missing System Attendant homeMDB attribute is a reason for this error.
Also, this error can happen when the mailbox database is corrupt or damaged.
Rebooting the Exchange Server may help to fix the error. However, rebooting the Server is not a viable solution as it causes wastage of precious working hours. So, we need to look into some serious troubleshooting methods for this error.
First, check if the default mailbox database is mounted. If not, mount it and check if the error disappears. Also, ensure that all Exchange services are running.
In the Exchange admin center (EAC), go to servers > databases, and check if the default mailbox database is mounted. If not mounted, select the default database, click ellipses, and click Mount.
If you have too many databases and some of them remain unmounted, better remove unmounted databases after exporting their data to other databases.
Note: The database may not mount properly if it is in a Dirty Shutdown state. You can check it using the Eseutil utility.
To verify this, use the cmdlets in Eseutil:
If it shows Dirty Shutdown State, you need to repair the database. You can perform a soft recovery (using Eseutil):
You can use ADSIEDIT to check the System Attendant homeMDB attribute. If this value is incorrect or missing, provide the correct value.
Finally, you need to check if the Exchange database or log files are corrupt. If they are corrupt, you need to repair them and recover their data. In case of minor corruption issues, you can rely on Eseutil utility to fix the issue.
For a hard repair without backup files, you can use:
But if the corruption is severe, you need to try professional utilities like Kernel for Exchange Server to export data from corrupt EDB files to a live Exchange Server. This tool repairs even highly corrupted databases and exports data to various destinations like Outlook PST files, live Exchange Server, and Office 365. This is a user-friendly tool and supports all versions of Exchange Server, including Exchange 2019.
The error ‘Cannot open mailbox /o=First Organization /ou=Exchange Administrative Group’ can happen due to various reasons, including Exchange database corruption. We have discussed various ways to deal with this issue. If this error is happening due to EDB file corruption, you can try Kernel for Exchange Server to fix the issue.