By default, Exchange 2007 retains disabled or removed mailboxes for 30 days after deletion.
You can change this setting on ??oMailbox Database Properties???, page ??oLimits???.
Disabled or removed mailboxes are listed in Exchange Management Console -> Microsoft Exchange -> Recipient Configuration -> Disconnected Mailbox.
You can see disabled or removed mailboxes by enter the following command in Exchange Management Shell:
Get-MailboxStatistics | where { $_.DisconnectDate ?ne $null } | select DisplayName,DisconectDate

But sometime we don?t see any disconnected mailboxes although they should be. In this situation we must run cleanup agent by enter the following command in Exchange Management Shell:
clean-mailboxdatabase ? Identity ?Mailbox Database???