First before trying to find any issues I would suggest that you download the latest version of avast, 4.7.827 and save it to your hard disk or do a manual program update.
What is your OS, I'm assuming win98 ?
If so then that is required to be able to intercept the email and scan it before it arrives in your inbox. Often this problem is becase of your firewall blocking ashMaiSv.exe, so what is your firewall and does it allow ashMaiSv.exe ?
Is your Earthlink secure encrypted email, e.g. SSL ?