Seems I broke the firewall on my mail server on Tuesday. I thought the mail was a little light for the last few days. A little further investigation showed that all of the messages were being rejected by the mail server because the sending address couldn't be resolved.

Connections initiated inbound on allowed ports were allowing, thus the server was accepting the connections, but when the DNS was checked as part of the anti-spam efforts, it failed as the response would be rejected by the firewall. The server would then reject the message. So, essentially, no e-mail for three days. I don't know that I missed it. Really.

Whoops. A few additions to the firewall, and incoming traffic due to established outbound connections would be allowed, whatever the port. Slight oversight.