Say my email address is
goeblog@worldcom.com
That would get spammed - alternative is goeblog at worldcom dot com - this negates web crawlign spiders
Doesnt stop spammers getting it by manual means but reduces the potential - if you use such web forums get a hotmail or yahoo address and use as an alternative account to youre main personal account - you can use that and just have it set to contacts from certain forums and no others. Or just use private message - its about protection.
ISPs dont really filter emails - cause mostly they also are the source.