Are you using Outlook Express? If so, into the firewall settings, is the following program allowed to connect?
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe (avast! e-Mail Scanner Service)
If you’re using only MS Outlook, make sure the avast plugin is not disabled into MS Outlook:
Outlook 2003 > Help > About > Disabled items
Outlook 2000 > Tools menu > Options > Other > Advanced Options > Add-In Manager.
Please, post more info 