I have a huge archive containing some giga of mail. Once, when I didn't have Gmail, I didn't filter them for spam, so now I have massive spam into it. Is there a way to let Gmail (or some other application) process that folder for spam, and filter all what it finds for trashing?
You could try POPing/IMAPing your email to an email client, using one of the various spam filter solutions.
Back in the days before I was 100% using Gmail (2008?), I was a big fan of SpamPal under Windows. But it looks now to be a dead project (although there are some recent kudos on the sourceforge page, so I think it still might work).
One caveat with many of these approaches: Effective spam filtering works (or used to!) thanks to IP black lists. As zombies are used to send spam, their IPs get flagged in the list in real time (through services like SpamCop.net and other blacklist contributors) and that's a very effective way to flag spammy mail (more effective than analyzing content, IMO). If your archive is old, the blacklist approach will work poorly, obviously because the blacklists evolve constantly (aka whack-a-mole).
Edit I agree that Gmail isn't going to work. I thought about forwarding the mails via Gmail. It probably wouldn't work, and if it would, you could possibly flag yourself for sending spam sending a giga of mail.
I would use a web based service like http://www.xeams.com/ It's 100% free with a fast set up, and you could just take all those emails and forward them to have it filter.