drwex | Needed: linux mail clue (postfix)

For some reason, outbound mail from my home server has stopped working. Mostly. Inbound is clearly working but outbound seems to fail silently. I can't figure out where the failure is occurring. Looking in /var/log/maillog I see outbound traffic lines like this:

Apr 1 17:01:01 hovir postfix/smtp[17121]: C738A74343: to=<ringel@****.org>, relay=smtp.mail.****.net[207.172.****.99], delay=1, status=sent (250 ok: Message 392262033 accepted)

(I have put **** in place of actual domain names/IPs for privacy purposes. The correct names/IPs appear in the log file.)

I interpret this to mean that outbound mail from my machine was accepted. Is there something I should be looking for to indicate failure?

EDIT 1: my current theory is that my ISP is blackholing me, presumably because it somehow thinks I'm a spam source. I'd love to find another outbound mail relay to point at so I can test this but then I'd have to figure out how to repointer postfix. It's been a lot of years since I frobbed this thing. :(

EDIT 2:

frobzwiththingz: i sent you two test messages, one from gmail and one from hovir.

EDIT 3: well, props to RCN, they didn't treat me like a freak and I got to talk to someone who does understand the basics of SMTP. He's never heard of Postfix, but he didn't blink when I told him it was running of an old Red Hat linux box. Ticket is opened and kicked upstairs to their engineering. Now we wait.