Go into Active Directory and load up the user you want. Go to permissions, advanced, the click on "Effective Permissions". Plop your BES admin ID there, and see if what it spits out... your account should have "Send As" writes.
Another way to test... log into a computer that has outlook as your BESadmin id. Open outlook, configure it for that user, and try to send an e-mail. If it sends, you have send-as writes