Save Sent Items in folder of a shared mailbox in Outlook 2007

We have a lot of users that access multiple mailboxes from within their own exchange account in Outlook. Attached as "additional mailboxes".

Mail can be sent and received from these mailboxes as long as the user has send of behalf of permissions. This is set in Exchange.

However, when a user sends an email on behalf of the additional mailbox, the sent email goes into the sent items of the person who has the additional mailbox attached (their own mailbox). And NOT the sent items of the additional mailbox.