uxbod: Thank you, I'll look into that.
cpm: Though they're set correctly on our host, we still resolve to our ISP's netblock and a few companies are blocking us based on that. My ISP wasn't willing to add a new record for us, though they don't mind us running a mail server on our business account.
We can get through to places like Yahoo and other somewhat difficult mail services, so this is a small problem overall.
I think I may have this working with what I did with this command:
It seems like it's trying a direct route first and then using the ISP's mail server if it fails, but I'm not 100% sure yet.
postconf -e fallback_relay=ourisp.mailserver.com:25