there are a few sites that do this.
they spam a list of people, telling them that they have a secret admirer.
in order to guess who it is, you have to put in that person's email address.
you are just spammed so there is no real address, so you arent' going to get a match - meanwhile, they are getting all of the addresses that you feed in.
they then spam those email addresses and it goes all over again.
there is a lot of overlap of emails, but in the end, they can get a large list this way.
whether or not they then sell that list, or ever had a legit site in the first place is questionable - but the first part is right.