The same way for newsgroups, programs exists that you can scoop through a lot of news groups and extract the sender email address and other emails from the messages.
If you do not want to do this your self there are plenty of people on the web that sells cds with millions of email addresses on for a couple of dollars. Does it make you angry that people are making money selling your email address for spamming?
Web and newsgroups are good sources for email addresses, but the people that do this are always looking for new ways to get fresh emails. So potentially everywhere you leave your email address you have a chance of increasing your spam. Webpages can give email addresses people enter on their webpages to others. But that is not thinking big, all places where your email appears in text have the potential to automatically be collected for spamming. For example if you are chatting on irc and you type your email in the chat, your email can be collected for spamming.
$ Editor: Joe. © Turtlemeat.com 2018