SMTP problem. Very strange.

We have a unique setup. First:

We have a Windows 2003 server with our own domain, which is cable.local. However, we are connected with a VPN router to another company's network that hosts our email for us. They use one set of email address, then use SMTP to give us what we want. For example, their address they automatically give everyone who signs up is soandso@go.com.

However, we need soandso@cable.org, so they set up the appropriate SMTP addresses and everything is fine.

The problem comes with our server. We have some services running that require the use of the native Windows 2003 SMTP service to send messages. Whenever the service attempts to send to the cable.org address, it fails, and sends an error to the badmail folder.

However, it sends fine to EVERY OTHER address, like yahoo.com and google.com. It even sends fine to the internal go.com address. The only addresses it does not send to are the cable.org addresses.

This had been frustrating the crap out of me for the past 3 weeks, because I can't figure it out. I think it's on their end, they think it's on our end. Does anyone have any ideas, whatsoever? Here's part of the text of an error log:

From: postmaster@cable.org
To: mrobison@cable.org
Date: Mon, 6 Jun 2005 16:16:44 -0500
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
boundary="9B095B5ADSN=_01C56AD7AF8BF4A800000012mainserver.cable"
X-DSNContext: 335a7efd - 4446 - 00000001 - 80040546
Message-ID: <3XCHj3FQQ00000022@mainserver.cable.local>
Subject: Delivery Status Notification (Failure)

This is a MIME-formatted message.
Portions of this message may be unreadable without a MIME-capable mail program.

--9B095B5ADSN=_01C56AD7AF8BF4A800000012mainserver.conne
Content-Type: text/plain; charset=unicode-1-1-utf-7

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

mrobison@cable.org