Phalanx was developed to be a powerful spam filter application for POP3
accounts. It can be used with any mail client and supports APOP (MD5) and
secure connections (SSL) for authentication on the mail server. Key features
of "Portable Phalanx" include: Blacklists and Whitelists; DNSBL filter that
searches the IP address of the sender in a DNSBL database. (in Internet)