Approximately 30 million users worldwide.

Spam detection stats.

Rules
Easy to extend Rules, weights and user-visible text are stored in text configuration files as much as possible, which the user (or sysadmin) can edit to modify or add new rules.

Wide-spectrum
Active Internet’s SpamAV™ uses a wide variety of local and network tests to identify spam signatures. This makes it harder for spammers to identify one aspect which they can craft their messages to work around.

Header Analysis
Spammers use a number of tricks to mask their identities, fool you into thinking they've sent a valid mail, or fool you into thinking you must have subscribed at some stage. Active Internet’s SpamAV™ tries to spot these.

Whitelists
Email sent from addresses on your 'white list' will automatically be skipped by SpamAV™ and will never be treated as junk mail. Place companies and individuals on this list that you expect email from, especially those who might have been accidentally tagged as Spam in the past.

Blacklists
Active Internet’s SpamAV™ Blacklist is a realtime DNS-based
databases of IP addresses of verified spam sources (including spammers, spam gangs spam support services, and compromised computers), designed to help email administrators better manage incoming emails. Active Internet’s SpamAV™ supports many useful existing blacklists, such as mail-abuse.org, ordb.org or others.

Email sent from addresses on the black list will be automatically be treated as Spam, and will be tagged as such. This is useful for filtering out individuals who persist in sending unwanted messages to you.

Razor
Vipul's Razor is a collaborative spam-tracking database, which works by taking a signature of spam messages. Since spam typically operates by sending an identical message to hundreds of people, Razor short-circuits this by allowing the first person to receive a spam to add it to the database -- at which point everyone else will automatically block it.

ClamAV...Anti Virus Engine
The main purpose of this software is the integration with mail servers (attachment scanning). The package provides a flexible and scalable multi-threaded daemon, a command line scanner, and a tool for automatic updating via Internet. The programs are based on a shared library distributed with the Clam AntiVirus package. Most importantly, the virus database is kept up to date.

Postfix Mail Program
An alternative to the widely-used Sendmail program. Postfix is fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.