as mac said, if you have a set time to check your mail, your phone will remain in passive data mode and the battery will drain rapidly and eventually shut off the phone..
The solution is to either diable the automatic mail checking feature, and/or manually disconnect the data service from your network prefs after the mail app checks your messeges.

Vision is always available, but you must dial in with your phone in order to get connected, and when you are finished with your mail, you may want to severe the connection to save your power.