Unfortunately the settings in the mail Shield don't cover this. I personally never allow autonomous action (regardless of program) this used to be simple as it was in the mail shield settings.
Now this option it is bloody hidden (buried) in the undocumented geek:area, hidden in the bloody Avast UI > Menu > Settings now type this 'geek:area' in the Search field without the quotes. When in the geek:area type mail shield, now you should have this screen (see attached image) displayed, I have arrowed what I have on mine, so any decision on actions is mine.
But I can't blame anyone if I get it wrong, so you do have to pay attention to what the alert says as simply and take care as opening it could put you at risk.
Without doubt the geek:area is a huge pain in the ass of any of the UI changes, and the sooner it is reversed the bloody better.