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