It’s like the system that is used by WordPress plugins like Spam Karma or Akismet. They have their own spam filters that filter out spam but also access blacklists and whitelists on the Internet that are created by a lot of users.

Bullgard Spamfilter integrates with Microsoft Outlook, Outlook Express and Thunderbird and requires a user account at Bullguard which can be created during setup (just enter a email address and password). A huge advantage over tools like Spamihilator is the fact that you do not need to change the account settings, e.g. the mailserver, in the email client that you are using, it works right out of the box.

One aspect that I personally do not like is that Bullguard adds a footer message to all mails that are send from your account, saying:

I use BullGuard Spamfilter to keep my inbox clean.
It is completely free: www.bullguard.com/freespamfilter

This can’t be unchecked in this version and is definitely a reason for me personally not to use it.