BES does not use .dat files. Toss that idea right out the window.
In your post with the CDO versions, it looks like you had older CDO on the BES. Make sure they are the same or the BES is higher.
I would doublecheck the permission requirements. Especially the send as/receive as permissions. Domain Admins have send as/receive as denied, so make sure the BESAdmin acct is not in the Domain Admins (but is in local admins).