It is highly unlikely that you received an infected email from your own computer.

  1. Avast would have given a warning on the outbound email.

  2. Your email address is simply in the address book or file on someones computer that is infected.

  3. Email that is sent from infecetd computers usually has the from address forged, this is usually an address found in the addressbook.

Doing this often bypasses some spam/av programs as your email address is in your white/safe list. I have removed my email address from white lists to ensure that it too is checked. This is more a spam issue as avast doesn’t care who sent the email it checks it.