I’ve struggled with iPhone Exchange ActiveSync and premature battery drain combined with overheating of the device. The only thing I’ve found that seems to be the answer for me is at:
This link explains an issue with Calendar items causing continuous contact from the iPhone to the Exchange Server and gives instructions for fixing the problem. It seems that the problem has a tendency to recur from time to time (as new calendar items are sent to you), but this process has worked to correct the situation when I run into it.
I would personally recommend additionally running the following reset afterward:
Settings > General > Reset > Reset Network Settings
Then re-sync with your mail server.