Mail are similar to ads found elsewhere on the web.
Some are banners at the top or bottom of your browser's window and change as you move from page to page. Others are simply links and images for you to visit, placed at various points throughout the service. These sponsorship ads are visible only by you. They are not in any way attached to messages you send, nor are they visible at all to your recipients. If you see an ad that interests you, go ahead and click on the banner, link or image.