We have a server in our dmz that for some reason cannot send email to
our domain.

Basically it's in our email domain (server.email.domain), and when I
ping our gwia (mail.email.domain), it gets the correct address. When I
telnet to mail.email.domain on port 25 from this server, I connect to
my gwia. I can even send generate an email this way and have it get
delivered correctly.

However, when the software set up on this server generates an email
(which has several of our email addresses in it), an undeliverable is
returned with this info:

"-------- Original Message --------
Subject: Delivery Status Notification (Failure)
Date: Wed, 16 Oct 2013 11:45:12 -0600
From: postmaster@server.email.domain
To: email_addy@otherdomain.com

This is an automatically generated Delivery Status Notification.

Unable to deliver message to the following recipients, due to being
unable to connect successfully to the destination mail server."

If I look at the attachment called "Attached Message Part" (which seems
to be another status type thing, I see this:

"Reporting-MTA: dns;server.email.domain
Received-From-MTA: dns;server
Arrival-Date: Mon, 14 Oct 2013 08:37:35 -0600

Final-Recipient: rfc822;<my_email_address>
Action: failed
Status: 4.4.7"

There are entries for every legit address in our system that is a
recipient of this email, all with the 4.4.7 status.

The reporting-MTA on this error is the from the sending system.

What gives? Any ideas?