Google AdWords use JavaScript and run locally on YOUR computer. This means that the JavaScript fetches ads based on a quick sweep of the text cached on the currently viewed page, based on frequency, so if the email says "cars" a bunch, then the ads will be related to cars, because the JavaScript then connects to Google's ad server and tells it "I want ads about cars." A simplified example, but again your computer is doing all the heavy lifting and Google is not "screen reading" your email themselves.