Of course, as it turns out this is a really bad idea. Not that I wouldn't mind getting ads showing me luscious beach-scapes and crystal clear oceans. It's just that very little of my mail actually pertains to anything I'm at all interested in. What's this? Don't believe me? The truth is, 90 percent of all mail messages I recieve on my 'public' accounts are spam, messages sent out by disreputable personages trying to hock their wares. So, if google uses this data to determine what I'm interested in, then all they'll find are subjects pertaining to insurance, home loans, pornography, viagra and various other unmentionables.
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I really don't need more pop-up ads targetting this stuff.