Hi and welcome, Joey,
It may depend in what you're using for mail. I use Eudora, and I'll see two things: when I start up Eudora, the "main" avast icon rotates to indicate it's checking out Eudora itself, the same as it would for any other program.
Then when Eudora checks for and downloads mail, an additional blue stationary icon appears (anyone know how to describe its appearance?).
Take a look at the headers of mail you've opened -- Eudora should be adding a couple of new lines to the heads, one showing its data-version and the other reporting (hopefully) "clean". If you see those, then avast is scanning mail just fine.
Hope this helps.
Best,
Mike