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While Exchange 2003 does have built-in support for realtime blacklists (RBLs), third-party applications are available that can extend that functionality to make it more versatile and useful.
One that has been around for a while is Martijn Jongen's Open Relay Filter, now in its 5.0.2 revision. ORFilter (as it's abbreviated by the author) is free, but donations to the author are encouraged. It works on Exchange Server 2000 or 2003 and Windows 2000 Server or higher.
Open Relay Filter supports several types of blacklists -- conventional DNS blacklists, such as ordb.org and spamcop.net, and a relatively new variety of blacklist known as SURBL – spam URI realtime blocklists (surbl.org).
SURBL blacklists maintain lists of URLs mentioned in spam e-mails -- any e-mail that contains a link to a blacklisted URL is strongly considered to be spam unless it's whitelisted in some fashion.
ORFilter also supports filtering by custom word lists. And, internal IP/SMTP blacklists/whitelists -- mail from specific SMTP servers, not just machines or e-mail addresses -- can be flagged as good.
When ORFilter flags a message, it adds a custom X-ORFilter header to the message to describe why it was bounced (keywords, blocked address, etc.). Each specific spam "symptom" is assigned a score value, and messages that exceed a given score threshold are marked as spam. The flagged e-mail can be deleted on the spot, rejected while still in SMTP processing, or admitted with a changed subject line or spam-relevant message headers.
Finally, all of Open Relay Filter's activity can be logged; if a message is rejected, the log will describe it in detail. A user doesn't have to track down the message itself and read the headers to find out why it bounced.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter and a regular contributor to SearchExchange.com.
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This was first published in May 2005