We have implemented SPAM filtering technology on our CentOS servers. The “Mail Abuse Prevention System” or MAPS is a not for profit organization committed to defending the internet’s email system from spammers. We have implemented the MAPS Spamhaus “Zen” technology. ZEN is the combination of all Spamhaus DNSBLs into one single powerful and comprehensive blocklist to make querying faster and simpler. It contains the SBL, the XBL and the PBL blocklist.
A DNS-based Blackhole List, or DNSBL, is a means by which an Internet site may publish a list of IP addresses, in a format which can be easily queried by computer programs on the Internet. As the name suggests, the technology is built on top of the Internet DNS or Domain Name System. DNSBLs are chiefly used to publish lists of addresses linked to spamming.
For more info you may visit http://www.spamhaus.org/zen/

While this is a very good front line filter, it will not eliminate spam completely.  We therefore also implimented Magic Spam Anti spam protection.

MagicSpam is a powerful Anti-Spam software that is installed directly on our mail server.   MagicSpam is based on the same anti-spam technology used by Large ISP’s and Telco’s.
The technology works by stopping junk mail directly in the SMTP layer offering better Zero Day Protection against trojans, bots, and other malware keeping both you and your customers more secure.  See more on the Magic Spam Filter.

The last line of defense against spam is your mail client. I.e. Outlook, Apple Mail, Etc.   These programs have learning technology included so you can “train” them to detect and filter spam.  There are also many other third-party spam filters that work very well with your local mail client software.