Since the highest performing ads are shown at the top of the page within Gmail, you can get a good idea of which Ad copy on the content network Google determines as superior. You can also see how contextual targeting works by sending yourself email and seeing which ads appear.
You’ll also notice that some ads appear even though they are not directly related to the content of your email. Pay particular attention to these ads, because Google deems them to be highly relevant for all users– even outperforming ads which are directly related to the content of your email.
When I talk about “relevancy”, I’m often speaking of “Click Through Rate.” If an ad gets a lot of clicks, Google will start testing the ad across a large section of AdSense websites on it’s content network. If the ad continues to do well, Google will syndicate the ad across an even larger portion of websites.