Problems sending to navy.mil

We are consistently having problems sending to navy.mil addresses. Your troubleshooting page reports "Host not found." I've checked their MX records, and all of them are up and answering. I'm able to send email to navy.mil address from GMail, and I even telnetted a session just now to see the complete conversation. Only Zoho can't seem to deliver to navy.mil.

From the troubleshooting page, if I select a pending message and try to resend it, it =very occasionally= goes through. Most of the time, it simply pauses a long time, then decrements the count of attempts remaining.

This has been a consistent problem for over a month, and only with navy.mil.

Can you help track down the issue?