Run the Mail Protection Wizard (Start Menu > avast! antivirus).
Choose other email client than MS Outlook/Exchange
Check Automatic protect all your email accounts
Check Protect the accounts created in the future
Finish the wizard
See this forum: http://forum.avast.com/index.php?board=2;action=display;threadid=2307
Oops, only in the Professional version. Or you can choose the ‘Silent mode’ into the Interned Mail provider settings and the file will always be sent to the Chest (default for answer ‘No’ at that settings).