Default email app is always on Before Android 2.2, the default email app would only show notifications if it was "off". That is, if I used Advanced Task Killer to kill it.

After Android 2.2, the default email app will show notifications regardless of me killing it.

Is there a way to get the old functionality back? 

I want it to always show notifications, but only when I tell it to be "on". I know that I can change the notification settings, but that's annoying and clunky.

If there is a good alternative email app that provides this functionaly and can sync with Exchange, that would work.
In my samsung i5500 I use a 'sync shortcut' placed on the home screen.   

It is a one click on/off sync. You can add the power control widget by long pressing the home screen>Add widget>Power Control  



Or there are some such one click utility in the market.