Mailing or advertising in the form of spam.
You can frankly say that many people are sick of spam, mailboxes are cleaned of junk probably even those who use mail rarely. How does the mailbox get into spam?
Yes, you correctly thought that they left their mail somewhere on the site. But believe me if you will have a spare box that is not the fact that there will not go spam. Especially if the box on the well-known service is simple. According to my reports, I see even how the most of my unknown emails come in garbage by the type of financial pyramids and the like. Here, approximately as with the selection of a password, a dictionary with many known names, names and the like is taken and is simply sent to everything through special mailing services. Also on free sale through the Internet there are programs for collecting and checking mailboxes. Offer bad people and ready e-mail databases, but often most of the boxes in them are non-existent. My personal opinion is that now there is little relevance in mailing lists, I clean my spam folders almost at once :)