One of the hot new things in anti-spam technology is something that's rather old. Yes, the Realtime Blackhole List is back. Only these RBL's aren't the old school DNS servers of yesteryear, these RBLs are maintained by the big anti-spam vendors and are completely proprietary. The new name is now, "IP Reputation," and that's showing up on the marketing glossies.
The idea is that you deploy a network of sensors (say, every anti-spam appliance you ship, or software-package installed) that relay spam/ham information back to home base. Home base then builds a profile of the behaviors of the incoming IP connections. Once certain completely proprietary threshold are crossed, the anti-spam vendor then publishes that particular IP addresses reputation to their service. The installed base then queries the reputation service on every incoming TCP connection to see how to handle that connection.
The response varies from vendor to vendor, but include:
- Outright blocking. Do not accept traffic from this IP address. The connection is terminated before any SMTP commands can be issued. Do not pass EHLO. Do not collect 220-ESMTP.
- Deferr. Issue a 421 error message. Smart mailers will attempt redelivery later. Bots are generally too stupid to try this and just pass on to the next address on their list.
- Throttle. Get very slow in accepting mail. Take a long time to issue 250-Ready statuses after SMTP commands.
The nice thing about IP reputation is that it is fast and cheap. Instead of having to lexically scan every incoming email for spamminess, you can just look at the source's reputation and block a very large percentage of messages. When we turned this on for our spam product a while back, the reputation filter blocked between 90% to 95% of all messages ultimately blocked as spam. Clean email is the single most expensive mail to pass since it has to go through every single stage of the spam/ham test pipeline, and blocking things earlier in the pipeline is a good way to shed load.
Not all optimizations are without side effects, and this one wasn't. The former student email server, titan, got itself 'greylisted' due to spam quantities. Around 50% of the message traffic into Exchange from this system was ultimately blocked as Spam according to the old anti-spam appliances we had (we'd routed its mail through the 'outbound' queue on those appliances so it wouldn't be subject to reputation tests, but would still scan email). As part of the migration of student email to OutlookLive.Edu, we set up forwards from the old cc.wwu.edu addresses to the new addresses. The spam-checkers on titan were of poor enough quality that enough spam got through to cause OutlookLive to start grey-listing Titan, causing mail to really back up on it.
That's not the only thing. Certain mailers managed by departments other that ITS here at WWU have managed to get themselves greylisted or outright blacklisted on these proprietary reputation lists. The one common denominator we've found is that certain specific UNIXy mailers do not apply their anti-spam processes to mail that is subjected to a .forward. At least, not without specific config telling it to scan that traffic. So if a person on one of these mailers has a .forward sending all mail into Exchange, the full spam-filled feed heads to Exchange and the reptuation of that mailer gets dinged.
Which is a long way of saying that, ahem:In this era of IP reputation, outbound spam filtering is now just as required as inbound.
Really. Go do it. It'll help prevent blacklistings, and that sucks for anyone subjected to it.