I think you're confusing the adverts displayed in the gmail interface with the sorts of ads that Yahoo and Hotmail used to run -- bloated signatures tacked onto

I think you're confusing the adverts displayed in the gmail interface with the sorts of ads that Yahoo and Hotmail used to run -- bloated signatures tacked onto the bottom of each email itself. I can't remember Google not showing text advertising through its interface, but do remember them being very different for not bombarding your contacts with advertising slipped into in the emails they sent.