Thank you for your replies to David and xmailer.
I have done what both of you have suggested me.
I have gone to avast! >mail shield>settings> and unchecked scan inbound messages.
HOWEVER: doesn't work. As soon I check for emails it happens again.
AND: when I go back to mail shield the box is checked again by itself.
REMEMBER: In the mail shield window there is no option such as "apply" or "save" or anything like that.
SO: what else would you suggest?
COULD IT BE: BUG in the avast! beta?
Thanks for helping me.