Blocking SMTP traffic based upon RBL....

[Forrest Aldrich]

There are probably dangerous consequences to doing something like this 
improperly; however, there is a project out there called PacketBL:

http://wiki.duskglow.com/index.php/Packetbl

It's for Linux only, unfortunately -- however I like the idea.   It 
interfaces with the packet filtering system and selectively blocks SMTP 
(port 25, configurable) traffic based upon RBLs etc.  

I wonder if there is a similar way to accomplish this with FreeBSD/ipfw...