Source - https://www.stellarinfo.com/blog/fix-exchange-deleted-mailbox-not-showing-in-disconnected/

How to fix “Exchange deleted mailbox not showing in disconnected”

Check is by using the PowerShell command below to see the disconnected mailboxes by opening the Exchange Management Shell and running the below

Get-MailboxStatistics -Database “DBNAME” | ?{$_.DisconnectReason -ne $null}

OR

Get-Mailboxdatabase DBNAME | Get-MailboxStatistics | ft

If your mailbox doesn’t show than the issue would be that the Exchange server for a reason it doesn’t process them correctly. To solve this one would need to run the command under the Management Shell to clean-up the database. It’s similar to the action Run Cleanup Agent we had in Exchange 2003. The process will scan the directory for disconnected mailboxes which aren’t yet marked as disconnected in the Exchange store and will update them accordingly.

Clean-MailboxDatabase “dbx01”

If you are using Exchange 2013 onwards the command Clean-MailboxDatabase has been replaced with the below

Update-StoreMailboxStat –Database “dbx01” -Identity “<>”

Now this can be a bit tricky, if you deleted the mailbox how can you get the GUID if it is not visible anywhere? For that you would need to get the information from the Get-MailboxStatistic by using the below format.

Get-Mailbox –Database “dbx01” -IgnoreDefaultScope | Get-MailboxStatistics | ft DisplayName,MailboxGUID

Before proceeding with the commands make sure to advice the users as for this to work the cmdlet needs access to update the mailbox database. This means that the Information store needs to be stopped or if this only affects one mailbox database, it needs to be un-mounted. Depending on the size of the database, the process will take a considerate amount of time apart from the fact that the effected users will not have access to their emails and incoming emails might bounce back to the sender depending if you have an alternative solution to queue emails at your internet provider. When the clean-up process is ready you would need to open the Exchange Management Console and in the disconnected area you should see the mailboxes.

You would then need to create a new Active Directory user to attach the mailbox to and user the Get-MailboxExportRequest PowerShell cmdlet to export the mailbox. There might be occasions that these do not show up and without tearing all your hair from your head, there is an alternate solution using third party Exchange Server Recovery applications.

Third party applications like Stellar Repair for Exchange. This application apart from being the best toolset for any Exchange Administrator, it can help a lot in such cases. Stellar Repair for Exchange is quite a powerful tool where it can recover deleted mailbox from all versions of Exchange Server. It can recover any Exchange EDB file, corrupted or not and you can recover anything from the mailbox store.

You can attach multiple mailbox databases and export directly to a live Exchange server or into an Office 365 mailbox. You can also save the recovered mailboxes to a number of formats such as PST, MSG, EML and PDF. It can process any size of EDB files and supports any Exchange Server version from 5.5 to 2019. It is the definitive bundle of tools for any Exchange Admin as apart from solving these issues, it will surely decrease by 90% the admin effort to recover and of course the downtime for the end user.