I just realised, and this could be red herring, that almost all of the domains I've administered were based in Australia. I suppose it's possible that the IP ranges I'm dealing with have a better reputation than those from other countries. I have administered a few domains from US companies and IPs, but they've often been based in known data centres which may help their cause. I can't really talk to the reliability of hosting a mail server on a consumer / small business IP in the US / Europe/ Asia. It's possible that all known, common IPs in these areas have a natural disadvantage when it comes to reputation. I suppose try running a tunnel from your server to a small VPS in a knwon data centre? Not ideal, but it may help.

It would be annoying if entire US/European/Asian ISP IP ranges were immediately blocked. We should have moved on from that for many reasons unrelated to email.