Greylisting seems to be the right method to effectively lower the spam levels your mail servers get daily. Read more about it on my Combating Spam with Greylisting article.
There are quite a few Greylisting implementations available for a wide variety of SMTP mail servers. So far I was only able to find one freeware Exchange 2000/2003 implementation (if you know of any other freely available tool please let me know).
JEP(S) is a spam filter which intercepts mail sessions before the mail is actually received. This is done by using several different technologies like greylisting, dynamic black- and whitelists and static whitelists. By doing this it drastically reduces the load on the mail server as the spam mail is never received. When JEP(S) suspects that a mail session is spam related it will send back a response to the sending mail server which indicates that the session has failed and that the mail has to be resent. The sending mail server will then queue the mail to be resent and retry at a later time. JEP(S) keeps track of this and when the mail is resent after a configured delay (normally 2 minutes) it will be allowed to be pass through to the mail system.
The benefit of refusing the mail before it’s delivered is that you have never received the email that you’re refusing. By doing this the sending mail system is informed while the session is still open that you’re refusing the mail. If, for whatever reason, something goes wrong with the re-transmittal of the email then the sending user will be informed in clear text about what action was taken on the mail.
Note: JEP(S) is the predecessor of a fine tool called Graylist, also by the same author.
JEP(S) currently combines three technologies to filter spam; greylisting, RBL’s (Realtime BlackLists) and RWL’s (Realtime WhiteLists). These three in combination gives you good spam protection with an extremely low rate of false positives in comparison with other spam filters. A low rate of false positives means that it’s unlikely that legitimate email will be treated as spam and blocked. And the block rate of your JEP(S) installation will of course vary depending on the volume and the type of email you’re receiving, but most of our implementations show an effective block rate of 94-98% on the greylist filter itself without taking into account the affect of the RBL feature.
The free version features:
As an add-on to JEP(S) it is possible to purchase a license to enable the advanced functionality. See author‘s website for more info on that.
To run JEP(S) you need:
Installation and usage:
To read more about JEP(S) please see the author‘s website
To download JEP(S) use THIS link
From my initial testing, using Greylist on Exchange 2003 production servers has dropped the number of spam messages being received by IMF (read Block Spam with Exchange 2003 Intelligent Message Filter) significantly.
As always, read the readme file for more information.
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