If you delete a AD user account it will automatically disconnect the connected mailbox from the AD account and add it to the Disconnected Mailbox list.
Mailbox will be listed until retention period of 30 days(unless otherwise set)
Get-MailboxStatistics -server <servername> | where { $_.DisconnectDate -ne $null } | select DisplayName,MailboxGuid,Database,DisconnectDate
Gets the list of Disconnected mailbox in the specified Exchange 2007 Server
Get-MailboxStatistics | where { $_.DisconnectDate -ne $null } | select DisplayName,MailboxGuid,Database,DisconnectDate