A quick search of the forum found this thread. The "Send As" Issue with Active Directory
This has been discussed at length. The thread I found shows a way to allow the rights to domain admins or other protected accounts but my take with this at my company is if you are a member of a protected account and have a mailbox, you need to be removed from that protected account and no one with a Blackerry who is a member of a protected group an send until they get removed form the groups. It is a best practice.