You probably receive emails from companies that you don´t remember giving your email address. Here is a small trick to find out how they got hold of your email. I have used it to catch a few companies who have sold my address. As far as I know it only works with Gmail.
The next time you register on a website use a + tag in your email address. You can use an unlimited number of tags. So if your email address is email@example.com and you want to register for, say, Decathlon´s newsletter, use firstname.lastname@example.org.
If you receive a promotional email from a brand a few months later, and you don´t remember signing up for their list, just checking the to field could reveal how they got your address.