We had this too. Blackberry Internet Service and BES were both setup on the handheld for the same account.
is what I looked at to get started. I had to go in and have the user delete his BIS account as it was configured to get his corporate email. You can't have the same email account configured to use BIS and BES.
We also had some users create gmail accounts and configured them to send messages to their BBs. This Link
will explain this fix in full.
I hope this helps.