I'm not sure about that. I think that serving relevant ads for emails doesn't need to associate the person with the email with the ad. Or does it? I guess when you open an email, they just extract the keywords out of it, and then run a search against their ads index to serve relevant ads. If that's the case, I don't think extracting keywords from email is different from scanning it against malware.

Besides, building an index is just necessary for providing email search. I'm sure outlook mail does that too, if their search is any good. I haven't used it, so I don't know.