Here's a useful article on wiki about email harvesting
en.wikipedia.org/wiki/Email_address_harvesting
Basically there are programs that can try various combinations of names with an email domain and chances are that at least some of them will be valid email addresses. Especailly if your email address happens to be something like 'firstname.lastname at googlemail.com'
There are other ways, including trawling forums (such as this one!) for data. Much depends on what level of protection the forum site affords their users.
Sometimes it’s just the small things that press the bruise isn’t it? 😢
