system
2
Don’t worry about a thing… just make sure that everything on real-time shield is on,
or you could just klick show details when you start avast to see if Real-time shield is : ON
if real-time shield is on your mail shield is on too 
Make sure that both options are checked in the mail shield tab
( Avast > Real-time shield > Mail Shield)
If the problem is still there you could always reset it back to the factory settings
(avast > Settings > Troubbleshooting > Factory settings tab)