Those working with IMAP4 mailboxes instead of POP3 often get ignored by various filter programmers - many can only handle POP3. The link goes to a filter that uses SpamAssassin against an IMAP4 server: emails are sorted out and thrown into a spam folder. The nice thing about it is that this filter doesn't need to be built into the mail client, but also not into mail delivery. Instead, you can simply run it outside of the mail program - it can run on any machine that has access to the mail account. Quite practical if you have your mailbox with a hosting provider and want to run the filter from your own server, but your mail program runs on a notebook (and you want to avoid downloading spam mountains while on the go, for example).
Oh, and the fact that the little program is written in Python is also not without its advantages - instead of SpamAssassin, you could also integrate SpamBayes, for example.