email addresses really are easy to get hold of, I have emails that have never been used that are on spam lists.
There are many ways to get them, if you us it to communicate, then it is on someone's email address book, if they become infected then their address book can be harvested and used for many purposes.
It depends on what the email address is, if from hotmail, yahoo or gmail, etc. then there are random name generators that will try and come up with a validaddress@adomain.vom, etc. If the first part of your email address is a word/name then they use a dictionary attack method using common words and names attached to hotmail, yahoo or gmail, etc. and they have a valid email address.
So as you may now see you don't have to give any password away for an email address to be harvested/generated. If you password isn't strong it won't take much to crack anyway.
Though why they would want to do this on a game site I have no idea.