Jun 24th, 2009

Block Spam on the Internet.

Filters are a joke. Content filters, anyway. The key is contextual filtering. Get an email server that does the following:

• Checks SPF ( implement this on your domain ) to block spoofing
• Require valid domain MX records
• Check dns PTR’s
• Use GreyListing ( spammers will only send once. If you tell them "try again later" they just drop the message. A real mail host doesn’t. )
• Use Tarpitting ( progressive response delays for each spam attempt ) to proactively slow down their infrastrusture. Remember they rely on speed and volume.
• Check the SpamHaus Zen list. Deny any flagged messages.
• Check the SURBL. This is the only time your mail server should actually parse an email besides looking for viruses.

That all will block ~98% of Spam. Filters are for the birds. For our average client moving to our mail server, the above methods will take the boss of the company from ~500 Spams a day to ~12. And forget big expensive paid services ( PSMTP, Symantec, etc. ). hMailServer is a free program, runs on Windows, and does all of that out of the box. There are a number of *nix based server softwares where all of the above is easy to implement.

And the key here is: if it fails, BOUNCE THE MESSAGE! Filters like Symantec’s, PSMTP, Gmail/Yahoo/MSN are problematic because they accept the mail before filtering it. More storage, you have to go in an actually empty your trash, etc. etc. If it’s Spam, your email provider does themselves and you no favors by filtering after acceptance. The <.1% of people who can’t pass the above checks will get the bounce, seek the cause, tell their mail admin to fix their setup, and go on about their business.

imho, if 90% of domains on the Internet could implement the above, Spammers would go out of business.