I had one of my servers put on the Real Time Blackhole list for a short
period because it was (at that time) allowing fairly open relaying. I had to
tighten it up and they took me off.
There are many mail servers out there that have a plugin for the RBL and
they automatically shut them off (I use VOPmail and it has that as a DLL).
You may need to find out (sorry don't remember the URL off the top of my
head, you will have to search for it.)
For more information about this list (including removal) go to: