Have you ever wondered how SPAM filters work?  Well, to start off, let me explain that there are 2 type of SPAM filters.  One type of SPAM filter resides on the client, while the second type resides on the server.  Most hosting packages (including ours) provides you with a server based SPAM detection tool.  The defacto tool provided on most servers is:

Yes.. you guess it… SpamAssassin. 

So how does this SpamAssassin actually work?

Well SpamAssassin works on a point based system.  It is much like the way our government regulates our Singapore driving license.  In Singapore, if you chalk up more than 24 points within 2 years, your license will be revoked.  Points are awarded based on the severity of the violation:

• 12 points for driving past red lights
• 12 points for speeding, etc…

SpamAssassin, as with other SPAM detection software, regulates SPAM in similar fashion.  They build a set of rules and start awarding points based on the contents of the email received. Mails scoring above a certain number of points will be branded as SPAM.  Check out the email below.  It shows an email which has been awarded a total of 22.4 points and has been branded as SPAM.  For an email to pass through the filter, it generally needs to score below 5.

Hmmm. so what are the rules and how will these rules help?  Well, as there are thousands of ways a Spammers try to circumvent SPAM filters, SPAM Assassin has also come out with thousands of rules to award the points to weed out the SPAMs.  If you are feeling really geeky, check out the rules here. (Different versions of SPAM Assassin has different rules… they have to upgraded constantly to cope with the new SPAMMING techniques).

Of particular interests is one rule for me.. the EMAIL_ROT13 rule:

ROT13 is defined as: The simple Caesar-cypher encryption that replaces each English letter with the one 13 places forward or back along the alphabet, so that "The butler did it!" becomes "Gur ohgyre qvq vg!" This test indicated an e-mail address was encoded using ROT13. This is normally done to hide the identity of the recipient used for list washing.

Hmmm… this explains why we sometimes receive all those totally unintelligible messages.  So these fellows are actually cleansing their email lists.. .and preparing for the next SPAM….