It has long been clear to me that the best place to stop spam is at an organization's SMTP server.
This is true for the following reasons:
Most spam has an invalid bounce address, so notifying non-delivery simply bounces to Postmaster, creating even more wasted bandwidth. Failing to notify non-delivery is a problem for false-positives. The SMTP server is the only place where spam can be stopped before entering your system.
The only possible feedback for spammers to void an address is from the SMTP server.
Spam that passes through your SMTP server into mailboxes incurs cost to your organization: storage, transmission, backup, deletion -- in all these ways spam costs you money.
The only way to minimize cost is to reject it at the initial point.
Easy browser-based setup (optional).
Uses your existing message transport and existing SMTP server.
Adds SSL/TLS support and Spam Analysis even for your non SSL aware MTA
Automatic whitelist -- noone you email will ever be blocked.
Early Sender validation -- Kills most spam before it ever reaches the server.
Advanced Virus Scanning with ClamAV and your File-System Virusscanner
Senders receive immediate notification if mail is blocked but no erroneous bounces are ever generated.
Redlist keeps an address off the whitelist.
No-processing addresses pass through.
Makes use of honeypot type spambucket addresses to automatically recognize spam and update your spam database.
Bayesian filter intelligently classifies email into spam and non-spam.
Community based grey IP list, Senderbase, SPF, DKIM support even if your MTA does not support it.
Supports additional site-defined regular expressions to identify spam or non-spam email.
Can optionally block all non-whitelisted email (for anti-spam zelots).
Mime encoded and other camouflaged spam is also recognized.
Automatically maintains the spam and non-spam databases.
Accepts whitelist submissions and spam error reports by authorized email.
Optionally rejects executable attachments from non-whitelisted (or all) addresses.
Practically no maintenance required.
Active user community and email list for support questions.
Optionally blocks no mail but adds an email header and/or updates the message subject.
Optionally uses community-based spam statistics to identify hosts that are likely to send spam or non-spam mail.
Individual users can be configured to receive all mail.
Shows detailed analysis of spam rating process for specific messages.
Option to forward a copy of every rejected mail to an address or to resend blocked Mails.
Can block spam-bombs (when spammers forge your domain in the from field) and fake Bounces
Keeps spam statistics for your site.
Can listen on more than one smtp port.