Can MessageStats report on message sent as another mailbox but in the context of the mailbox which the message was sent as?
This functionality is not currently available in MessageStats. Enhancement request TF00200974 has been submitted to Development for consideration in a future release of MessageStats.
Microsoft has changed the way they write messages to the tracking logs in Exchange 2010 that were Sent As another mailbox. In eariler versions of Exchange when a message is Sent As another mailbox the Sender-Address field in the Tracking Log is populated with the mailbox that is being Sent As. In Exchange 2010 when a message is Sent As another mailbox the Sender-Address field in the Tracking Log is populated with the mailbox that is actually sending the email and the Custom-Data field is populated with PurportedSender=Mailbox_That_Is_Being_Sent_As. MessageStats uses the data in the Sender-Address field to determin who sent the email.
In Exchange 2010 the sender field is the user that sent the email and no longer the mailbox that was sent as.