I'm not sure about the "Internet Mail" Scanner yet since I have a yahoo account.
Does Yahoo use SSL connections? If so, avast can't scan them without 3rd party software.
If you're not using email programs (pop3), but you're only using webmail, then you could not run the Internet Mail.
But, if you let it running and configure it well, you can detect if your computer is sending spam.
However, the option to configure each individual module/scanner is a good setup.
Sure, one of the great advantages of avast.