A couple years ago I created a new gmail, with a very long address. Didn't tell anybody, sign up with ANYTHING. Just parked it and let it sit dormant. Within a DAY I had my first spam.
Google is an ad company and it's entirely reasonable to assume they are simply selling lists of their own email addresses. Same with Yahoo! Mail! Which! I! Ran! The! Same! Experiment! From! With! The! Same! Results!
Ample but because it was one of the gmails I could create with an invitation generated by my own account, this was before you needed ID or anything but a link BTW, it would reveal my true life name which is my "mother" gmail address. I am one of those ass holes who got firstname.lastname@gmail.com as their address. I had a guy offer me $15,000 for my gmail.
Google is an ad company and it's entirely reasonable to assume they are simply selling lists of their own email addresses. Same with Yahoo! Mail! Which! I! Ran! The! Same! Experiment! From! With! The! Same! Results!