Diagnostic-Code: smtp; 550-RFCI-002 - oneparticularharbor.net is listed in the
Oh, joy. That's just want I want to start my day with, finding out that I'm on someone's blacklist somewhere.
Turns out our DNS records for our mail host were (mildly) screwed up... that was all. Not only that, but the admins at rfc-ignorant.org were extremely responsive and polite, and helped me get things corrected literally in a matter of minutes.
I mean... wow. Here I was expecting to have to slog my way through some extended "No, I'm not a spammer! " handshaking, and it turns out that it was just some technical glitch. Not only that, but someone who doesn't know anything about me was willing to take their time, lend a hand, and help me get things fixed up.
We'll know in a few minutes whether or not the DNS changes worked; they look good, so everything should be fine, but you never know. Whatever happens, hats off to the folks at rfc-ignorant.org for being just, you know, such nice guys.