Spit or Swallow the Spam

I receive about 250 spam messages each day, but virtually none of them ever reach my mailbox thanks to a rather aggressive spam filter.

Our mail server and spam filter setup is comprised of the following applications:

• Qmail - the mail transfer and mail delivery agent. It's the mail server software.
  
• Spamdyke - performs connection-level graylisting and blocking of bad IPs. Graylisting is a relatively new mechanism that pretends that there's a temporary error on the mail server the first time it encounters an email address. A proper mail server will re-transmit the mail within minutes, but spam software doesn't, and the email will spam therefore never be received.
  
• Qmail-Scanner - scans the contents of each incoming mail on its own, and more importantly invokes a variety of mail scanners to catch viruses or spam.
• SpamAssassin - scans the contents of each incoming mail for spam characteristics.
  
• Pyzor - scans the contents of each incoming mail for spam characteristics based on a spam "signature." The signature is a "compressed" version of the mail, and if this signature can be found as marked by spam on a central server, then it means the message is spam.
  
• Razor - applies the same principle as Pyzor.
  
• DCC - applies the same principle as Pyzor.
  
• ClamAV - a very capable virus scanner.
• TMDA - "tagged mail delivery agent," which assumes that all senders are spammers until they've confirmed that they're sending legitimate mail; a confirmation that they only need to provide one time.

Spamdyke and TMDA are built on the idea that spam won't stop, but you can make it prohibitively expensive for spammers to spam you, because they'll have to monitor your connection to see if their spam message was successfully delivered.

TMDA and the various scanners are highly effective and are sufficient for a near-complete elimination of spam messages. However, network bandwidth is a problem when your network is bombarded with spam messages. This is where Spamdyke can help, because it sits in front of the mail server listening to incoming connections, and is capable of rejecting spam messages before the contents ever reach the server.

The setup may seem a bit involved, but when each of the antispam measures have been installed, only Qmail-Scanner, Spamdyke, and TMDA require special setup. SpamAssassin, Pyzor, Razor, DCC, and ClamAV are all detected and invoked automatically by Qmail-Scanner. The result of this setup is that with very few exceptions, no spam finds its way to my mailbox.