I am on a BES at work, and I had my home email setup on the sprint BIS. But this week the second mailbox on my screen disappeared. I still receive my personal email into the main mailbox, but I can find no way to send email using the personal account.
When I open a message that is from my personal email, at the top it says "received using: Unknown". Yesterday I logged in to the Sprint BIS site and had it resend my service books to my phone. I got the confirmation message on my phone, but the mailbox never reappeared.
At this point I would recommend checking with your BES administrator to see if they pushed a new policy recently. With that one service book stating that it is disallowed it sound like there is something going on with the BES policies.