You can not switch to Simple MAPI. We are currently evaluating to enhance VPE, so the sender can be specified for Extended MAPI. But this will not work reliable, because from the information available through the internet, there are several problems (which is the reason why we had disabled this option for Extended MAPI):
It does not work with several mail clients.
Some other mail clients will show security alerts.
If using the Exchange server, you will have problems with account permissions.