I'm curious about what a program like this would be used for, because the only use I can think of is spam. I don't like the idea of helping anyone do that.

But it's possible you could want this for some other reason, so you may as well look at HTTP and FTP clients, where you should find some answers about getting contents of web pages and parsing the results. From there you could either look for 'mailto' or use some inexact regex to find things which look like email addresses.