A user not being able to send email from their BlackBerry is a nightmare - often the user is (like all users) impatient and in need of a quick resolution, especially if they are in a remote location. The inability to send email can also be a royal pain in the arse to pin down to a specific cause, and all too often you can spend an hour or two trying to resolve what has happened.
In addition to my thread covering the
'Send As' decision tree, there are a few specific KB's which can help you out. I've listed these below:
Unable to send email messages because the Send As permission has been revoked
Using the SetSendAsPermission tool
Transaction error messages
The BlackBerry device user is unable to send or receive email messages
Administration accounts in protected Microsoft Windows Active Directory groups
If you're still screwed after this, you know where to find us!