DNS Blacklists
New in Domino 6 is the ability to integrate DNSBLs (DNS blacklists) into your anti-spam arsenal. DNS blacklists are databases of IP addresses known to be a source of spam. There are many free and commercial DNS blacklists to choose from, some specialize in listing known dedicated spam servers (such as Spamhaus), others dedicate themselves to listing open relays (i.e.: ORDB).

When a foreign SMTP server connects to a Domino system using DNS blacklists, the Domino server will query each blacklist server (via DNS) to see if it contains an entry for the connecting SMTP server. If any of the blacklists return an entry, Domino can reject, log or log and tag all mail from that SMTP server as spam.

Note: For each blacklist you add, Domino will have to do an additional DNS query before accepting mail from an SMTP server.

To add blacklists to your Domino configuration, open the Administrator client and select the Configuration tab. In the left pane, select Messaging -> Messaging Settings. In the Messaging Settings tab, select Restrictions and Controls -> SMTP Inbound Controls and scroll down to the DNS Blacklist Filters section. Here you can enable the DNS blacklist feature, add DNS blacklist sites (separate each server with a ";"), tell Domino what actions to take when a connecting host is found in a blacklist, and create a custom SMTP error response for any messages rejected due to blacklisting.

A basic blacklist strategy
When starting out with blacklists, add two or three well known blacklist servers and add the following entry to your notes.ini file: SMTPExpandDNSBLStats=1 . This will allow Domino to display how many messages are being caught by each of the DNS blacklists. For the first week or so, select "Log only" as the desired action to ensure you don't end up rejecting mail from clients or partners by accident.

Check your logs and statistics regularly. If your second or third blacklist is returning next to no responses, you should drop it from the list. Play with the order in which your blacklists appear and compare the statistical results in Domino Administrator.

I recommend using dnsbl.sorbs.net (at http://www.dnsbl.sorbs.net ), relays.osirusoft.com (at http://relays.osirusoft.com ), and relays.ordb.org (at http://www.ordb.org ) as they are currently free and comprehensive blacklists covering the first four of the five sources of spam listed above.

[We should say we've got very mixed feelings about blacklists here at ZATZ. We've been looking at this issue in detail for both DominoPower and OutlookPower and are worried that the cure may be worse than the disease. The problem is you offload responsibility for your incoming "flow" to a third party. Some of the blacklists are quite reasonable and remove true spammers. Other blacklists will block any mail from someone who's gotten a single complaint. If someone doesn't like you or your company, a single email to that blacklist will forever alter your email flow. If you choose to use blacklists, you'll need to monitor their behavior VERY carefully. -- DG]