I don't believe there are any ads yet. Mark mentioned once that he wanted to get the site out of beta mode before pursuing them. There are several email services that insert footer ads. Mark has managed to trap a lot of them, but it's still on Trello:
If you can forward him a copy of the offending emails, I think it makes it easier to add to the filtering. Besides Avast and Inbox.com mentioned on Trello, he's also done Juno and a couple of others. It might be worth asking if he can post a list of all of them. Or maybe just add those on the Trello card, too.
Duane
Duane