Probably you have a conflict in the port 25… Can you see if any of these threads will help you?
http://forum.avast.com/index.php?board=2;action=display;threadid=7751 http://forum.avast.com/index.php?board=2;action=display;threadid=7610 http://forum.avast.com/index.php?board=2;action=display;threadid=7314 http://forum.avast.com/index.php?board=2;action=display;threadid=6975 http://forum.avast.com/index.php?board=2;action=display;threadid=6763 http://forum.avast.com/index.php?board=2;action=display;threadid=6760 http://forum.avast.com/index.php?board=2;action=display;threadid=5895 http://forum.avast.com/index.php?board=2;action=display;threadid=5873