You probably receive newsletters from too many companies and in some cases do not remember ever giving them your email address. Here is a small trick to find out how they got a hold of your email. 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 firstname.lastname@example.org and you want to register for, say, Decathlon ́s newsletter, use email@example.com.
If you receive a promotional email from a brand a few months later, and do not remember signing up for their list, checking the to field could reveal how they got your address.