Exchange Server 2007 doesn’t allow us to purge the disconnected mailbox. In order to remove one or multiple disconnected mailboxes we can be performing these steps:

Listing all disconnected mailboxes

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

Removing a single entry

Remove-Mailbox -Database <Database-Name> -StoreMailboxIdentity <MailboxGuid> -confirm:$false

Removing all users at the same time

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

Now that we have all disconnected mailboxes in a var, we can run the following cmdlet to remove all of them:

$users | ForEach { Remove-Mailbox -Database "Mailbox Database" -StoreMailboxIdentity $_.MailboxGuid -confirm:$false }