I intend to lodge a formal complaint with the international email authorities concerning this curious BigPond behavior, which is not in keeping with the spirit of the Internet.
Here's the precise technical data [in which I've obliterated part of the email address of my intended receiver] confirming the blacklisting:
X-Postfix-Sender: rfc822; email@example.com
Arrival-Date: Thu, 21 Jun 2007 12:45:37 +0200 (CEST)
Final-Recipient: rfc822; firstname.lastname@example.org
Diagnostic-Code: X-Postfix; host extmail.bigpond.com[184.108.40.206] said: 451 Mail from this IP address blocked due to DNS block list. (in reply to MAIL FROM command)