You won't find a GUI mailsever, that's pretty much an oxymoron in the unix world. What you can find is something like webmin to configure the mailserver.
Also you'll need more than just a mail server for that, unix is like a puzzle without a predefined picture, everything is broken down into small, coherent pieces that you link together however you want. Basically you'll use something like fetchmail or getmail to download the mail from your ISP then it'll deliver to the SMTP server of your choice (Postfix is what I use) and any filtering will be done after that. Depending on how you setup the rest will determine how you can do the filtering, using Courier-IMAPd is pretty simple and you can plugin procmail for filtering pretty easily. I use Cyrus IMAPd because it's a real speed demon, but it's also a bit of a pain to setup, you get to decide how much time you want to spend on it. But for 3 years Cyrus might be overkill unless you have huge mailboxes.