I use zen.spamhaus.org on my server, so you're welcome to send a test message directly to me off list and see if it bounces or not (you will get greylisted first for 60 seconds)
Sent a test message. Here's what's in maillog: Mar 14 11:00:28 mailserver sendmail[25108]: n2EF0LTt025094: to=ned@unixmail.co.uk, ctladdr=jep@obrien-pifer.com (503/503), delay=00:00:07, xdelay=00:00:03, mailer=relay, pri=120391, relay=smtp-server.carolina.rr.com. [75.180.132.33], dsn=2.0.0, stat=Sent (Message received: 20090314134107.MZQS7442.cdptpa-omta01.mail.rr.com@mailserver.obrien-pifer.com)
Been several minutes and no bounce. Guess that's a good thing.
It shouldn't if everything is configured correctly. As I said above, to me it looks more like the receiving server is misconfigured. That might be intentional, who knows, but they will get FPs as a result from anyone like yourself who correctly relays mail through their ISP.
That would stink, but appears to be the case.
Thanks, James