In the user control panel, by default our email addresses are hidden from public view. There are many other less sophisticated message boards that do not have such features, or the user neglects to hide their email address properly. What happens then is, little script kiddies run a program that requests a webpage from a site - and to the web server it would look the same as if the request came from a web browser - after capturing the HTML, another tool is used to scan the HTML source for email addresses. If your email address was blatantly printed out, even if in a URL, the scanner will pick it up and add it to a database.
Those buggers run these scripts repeatedly and mine a website for email addresses of people who visit that site.