These are online databases which contain sites that are known to send spam, or are known as open relay sites. DSMTP can check incoming connections against the list of servers on these databases in order to see whether they are listed, and then take various actions such as disposing of the message or rejecting it. This allows you to cut down on a lot of SPAM, and you don't have to be the one who has to update the list of "bad" servers each day :) as it's all automatic.

Occasionally there will be circumstances in which some servers will be in these databases, but you still wish to allow messages through from them, so we have also recently added exemption rules in order to allow for this. Please also note that in order to take advantage of these checks, DSMTP has to check each incoming connection, so this could cause time delays on some servers

ORBS was closed down a long time ago, and MAPS RBL has now moved to a subscription based service but there are plenty of free RBL's around. Just do a search on google

We have recently added generic functionality so that you can point DSMTP at any RBL server. You need to be running at least version 3 to use this.

Please let us know if you know any good RBL sites that are free and we will add them to our list.

You can try
Some RBL servers

rbl_vanish <host>
This causes DSMTP to accept all mail from the listed server as if everything is normal, but then silently delete every message it has recieved.
rbl_ban <host>
This causes DSMTP to give 500 errors when remote server connects.
rbl_reject <host>
This causes DSMTP to give 500 errors to each recipient given by the listed server.

A real examples
rbl_ban bl.spamcop.net

You can also use these exemptions
The rbl_except_mail and rbl_except_rcpt cannot be used in rbl_ban mode
rbl_except_ip <rbl-domain> <ip> - specifies ips to exempt from the rbl database
rbl_except_mail <rbl-domain> < address > - specifies mail from addresses to exempt
rbl_except_rcpt <rbl-domain> < address > - specified rcpt's that are exempt