MS MVP的牛文 ---- Removing disconnected mailboxes in Exchange Server 2007

From: http://msmvps.com/blogs/andersonpatricio/archive/2007/10/08/removing-disconnected-mailboxes-in-exchange-server-2007.aspx
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 }

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Daniel said:
Just a little improvment to remove on all databases of the server:

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

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