SquidGuard is a URL redirector used to use blacklists with the proxysoftware Squid. There are two big advantages to squidguard: it is fast and it is free.
Dovecot
Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It's fast, simple to set up, requires no special administration and it uses very little memory.