I figured out my SpamAssassin RBL problem... My first entry in
/etc/network/interfaces had an error in the name-server line.

Everything else on the machine worked because the subsequent entries
were correct. From a stand-alone perl script Net::DNS would work, but
SA, which uses Net::DNS, apparently won't go to the second entry.

It's a known issue when the first name-server entry is 127.0.0.1. My
first entry was a different error, but had the same result.

Steve