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I'm using version 8.4 on debian 7 with a sql backend.

Our mx handles several email accounts using multiple domain names.  One particular domain name isn't working with email.  From the johncenamail.com domain, and email address associated with it can send, however it cannot receive.  Any time an email is sent to the problem domain from an outside service provider such as gmail, yahoo, and zoho... we get a service delayed message like so:

This message was created automatically by mail delivery system.

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

The original message was received at Wed, 27 Mar 2013 11:33:58 -0700
from validemail@zoho.com [validemail@zoho.com]

    ----- The following addresses had fatal errors -----   
   [Status: Error, Address: <brad@johncenamail.com>, ResponseCode 421, , Host not reachable.]

Message will be retried for 4 more day(s)

Thinking my dns is wrong, I decided to use a web based dig service to check.  I will use the acutal address here to help in troubleshooting.  First, I wanted to see the mx record for johncenamail.com.  I used http://mxtoolbox.com/
It reports the correct mx for johncenamail.com.  I then used the same website to see if mx.joplinservers.com resolves to the correct IP, and that is fine as well.  Email accounts using other domains can send and receive fine, so obviously the firewall is not an issue as well as the server's ability to send and receive in general.

Out of curiosity, I wanted to see if the problem domain can receive emails from other domains hosted on the same server.  This does indeed work.  It's not simply because the DNS is answering it's own querries either, because the above website also reports the intended information... and when I dig and ping from my personal computer, it matches exactly the website information.  My personal computer uses different DNS's than the server... along with being on a different ISP's network.

Thinking this may be a classic case of DNS caching, I put this problem on the back burner... TWO WEEKS ago.  Any cache/ttl's should have expired long ago.

I'm puzzeled.  Any suggestions would be very much appreciated.