disable Avast's email scanning. It is a waste of system resources and no end of problems.
It sure is not the end of problems, more likely the start of even worse.
And since when is protecting a system a waste of resources?
1] As is the case with al av applications.
2] Not true at all. It depends on the settings a user has chosen in Avast.
3] Again not true. Avast will only remove the the infected message if you use OE. Other email clients who are using just one file for all the messages may loose every message but surely will not stop functioning.
The one who told you this really should spend some time on research before saying anything or just keep his mouth shut if he (she) don't know what he (she) is talking about.