Users are unable to send mail from BlackBerry Enterprise Server (BES) activated BlackBerry devices? Do you find err_submit_mail messages in the message agent logfiles?
In that case it is probably just a case of missing “Send As” permissions for your BES service account on the user accounts.
The BES submits mail, sent from BlackBerry devices, to the Exchange MTA for delivery. It does this while running in the security context of the BES service account and submits the mail as the user who created the mail. Without the “Send As” permission being set on the user’s account for the BES service account Exchange declines the mails.