Gmail’s personalized ads are nothing new. Back in 2011, Google announced that Gmail would no longer show just ads related to the message you’re currently reading, but also ads related to topics you’re interested in. “For example, if you’ve recently read a lot of messages about cameras, maybe you’d like to see an offer from your local camera shop.”
I’ve recently noticed some Gmail sidebar ads slightly different than the experimental format from 2011. You could hide the ad by clicking the “x” icon and even check other ads by clicking the “more promotions” link. The “sponsored promotions” page actually has a permalink: https://mail.google.com/mail/#pinbox (first spotted a few months ago). If you don’t see any promotion, opt in here.
The ads look like regular emails, although features like “reply” or “mark as spam” aren’t available. “It’s a new type of ad you can save to your inbox or forward on. If you dismiss this ad, you won’t see it again.” Ads have permalinks like: https://mail.google.com/mail/#ad/76002.
