I've found that my server has been blacklisted on 0spam (http://0spam.fusionzero.com/) and the reason given is this:
v=spf1 mx -all Return codes were: 127.0.1.0
I've checked all my configuration settings with respect to spf and they all appear ok.
I just cannot see where the class C address is being picked up from the mail server.
I'm using postfix as my mail server.
Any help would be super appreciated, as this one is confusing the hell out me.