Greetings,
I'm running PunBB 1.2.6 (nope - haven't updated yet) and I got a '553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)' message recently.
I think at least one person is subscribed to this forum, so I take it that is what triggere the mail. Based on you previous comment, I'm guessing it was caused because the first line of text in the post was terminated by a ";". What do you think?