Disconnected Mailboxes

Disconnected mailboxes are both usefull and not, in my opinion but if you are sure you don't want them then you can start by listing them in the Exchange Management Shell:

Get-MailboxStatistics | where-object { $_.DisconnectDate -ne $null } | Select DisplayName,MailboxGuid

If you are satisfied these are all to go, you can mark them for deletion (which I belive will occur the next time you server performs its rutine database maintenance):

Get-MailboxStatistics | where { $_.disconnectDate -ne $null } | select MailboxGUID | ForEach { Remove-Mailbox -Database "Mailbox Database" -StoreMailboxIdentity $_.MailboxGUID -confirm:$false }

Becarefull with the -confirm:$false part and also note that you will need to substitue your own mailbox name for the mailbox database which stores the disconnected mailboxes

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