>Your phrasing is misleading. Google do not "peruse" emails. Automated software scans emails for keywords and phrases and tries to add adverts that might be relevant. Humans are not perusing them. But that is not the problem with 3rd party mail providers anyway.

The parsers can't be blind. Some way to decide which words are keywords or keyphrases must exist, even if that's by catching similarities with those in the ads, or with words provided by advertisers. Without some human input, my guess would be that the targeted advertising wouldn't work at all, because the keyword list would begin with "the", "is" and "and".

Of course I don't mean that the google is a mechanical Turk - just that at some point (in the past, during the development of the process, at spot checks) some humans must read some emails and compare them with the proposed set of matched keywords.