There was a submission[1] here on HN a while back which went into some detail about the how and why. He got an email from a site that he started to sign up for,

There was a submission[1] here on HN a while back which went into some detail about the how and why. He got an email from a site that he started to sign up for, but did not hit submit. Using the browser dev tools, he saw that it recorded the email that he put in. [1] https://news.ycombinator.com/item?id=23384452 https://news.ycombinator.com/item?id=23384452