What is your OS, win9x/ME ?
What is your firewall ?
Does it allow ashMaiSv.exe internet access ?
This is a requirement in order for avast to scan your emails before they arrive in your inbox.
The server address for both POP and SMTP should be set to 127.0.0.1
The POP server name set to: username#pop.isp.com (use the actual username and ISP info)
and... SMTP server name to: username#smtp.isp.com
You can go into the Help file within the Avast Scanner and look up Manual configuration. It will take you through it step by step. It is not hard to do.
Check the avast help file for Setting up mail Protection. Switch of your email client and run the Mail Protection Wizard - Or you can set it up manually.
C:\Program Files\Alwil Software\Avast4 - look for, ashPopWz.exe - double click this and it will run the Wizard
Or see this thread for further information and advice
User's FAQ - Mail Protection.