Yes, this is another open source application with a goofy name. Fluffy is an anti-spam program that works at the server level. I first heard about Fluffy from an article at http://www.windowsitpro.com/.
We implemented Fluffy in December 2004 at both The Community Foundation of Sarasota County (CFSC) and Vangor Computer Technologies (VCT). All incoming email goes through Fluffy, first. If the incoming email message looks legit, it passes it on to the Exchange Server. It took a tremendous a burden off of both Exchange Servers! Additionally, we cut the spam down to less than 10% of the original volume.
Fluffy uses the following techniques for deflecting spam:
- DNS Black Lists
- Gray Listing
- Custom Black List and White List
- SMTP Protocol Compliance
Fluffy blocks over 50% of spam by "gray listing". This is when an incoming mail server is unknown, so Fluffy pretends that it's having technical difficulties. Therefore incoming mail server is unable to deliver the message. The vast majority of legitimate mail servers will retry delivery at set time intervals for up to 72 hours. When the incoming mail server attempts delivery 15 minutes later, Fluffy will receive the message successfully. However, most spammers are too impatient to attempt delivery 15 minutes later. After all, they are trying to send thousands of messages an hour. They don't have time to come back to some apparently unstable mail server 15 minutes later. For some reason, "gray listing" is considered a controversial method of blocking spam, although I'm not sure why. It's very, very effective.
The bulk of the remaining blocking is performed by blocking email servers that have been known to carry spam (this is known as "black listing"). When Fluffy starts receiving a message from a known spam server, it stops the communication in midstream without sending an NDR error message back to the sender. This saves a lot of time and avoids the possibility of spammers relaying spam off of your server via NDR messages. This is also what took a load off of our Exchange servers.
Fluffy not very good at content-based blocking. Another words, it isn't really designed to block messages that contain the word "viagara". To me, this was a good thing. Content-based filtering is never a 100% game. Inevitably, non-spam messages get blocked.
Here's a real example of an innocent message getting blocked by content filtering. There's a local pub on Cattlemen Road called "The Cock and Bull Pub", and Stan Schultes and the rest of the Sarasota .NET Developers Gang meet there once a month after their regular meeting CFSC. Stan reported that he had to be careful mentioning the actual name of the pub in email messages because he had problems with his messages blocked by some spam filters because the pub name contains the word "Cock".
Colin Boyle, of the Selby Botanical Gardens, mentioned a similar problem. Selby has botanists on staff that communicate with other botanists around the world. The botanists sometimes discuss the reproductive processes for various plants via email, and unfortunately some of the terms correspond to the reproductive systems in humans. Inevitably, this caused problems where content filtering blocked innocent email, which hindered productivity (no pun intended). Content blocking is just unreliable.
Fluffy also integrates with AVG anti-virus so that viruses and worms are removed from the incoming mail before it gets to your mail server. This has allowed CFSC to avoid having to install an anti-virus agent for Exchange Server, which saves money and makes Exchange run better. You can read more about this on my AVG Anti-Virus page.
Colin Boyle and I did a joint presentation about Fluffy back in March 2005. We wrote up some good material and made other resources available. I've included these materials in a ZIP file that you can download here.
To learn more about fluffy, go here: http://smtpfilter.sourceforge.net/