SpamPal sits between your email program and your mailbox, checking your email as you retrieve it. Any email messages that SpamPal considers to be spam will be "tagged" with a special header; you simply configure your email client to filter anything with this header into a separate folder and your spam won't be mixed up with the rest of your email anymore!
But how does SpamPal know what is spam and what isn't? Well, it uses what are called DNSBL lists. Patterned after the famous MAPS RBL, these are lists of parts of the Internet that in one way or another facilitate spam. Any email you get from a machine on one of these lists has an increased probability of being spam. Some ISPs already block all email from machines on some of these DNSBL lists, but many do not. This is where SpamPal comes in.
You can choose to use any or all of the freely-usable DNSBL lists; SpamPal will look at the machines each email message passed through on its way to your mailbox, and if any of those machines is on one of the DNSBL lists you have chosen then that message will be tagged as spam.
Because not everyone who uses a machine on a DNSBL list will be a spammer, SpamPal has a powerful whitelisting feature that allows you to ignore DNSBL listings for certain senders or for parts of the Internet.