Why are people emailing me getting their mail returned with the warning, "...refused by blackhole"?

Solution:

BuffNET is currently using several methods of filtering SPAM - the latest is a service called "Blackhole".

In a nutshell, the Blackhole services keep a database of known mail systems that have an "Open Mail Port", meaning anyone anywhere can send email through that system...

Meaning SPAM-houses can use them for sending spam to...

Well, anyone. Anywhere. Including you.

If your mail was rejected for this reason, you should forward the bounce message to your server's administration. They can then take steps to close off the mail system from spammers, be retested and cleared of the blackhole list.