External Email Senders are Getting Bounce Backs while other messages go through fine

Over the last several weeks external email senders have started to get the following message when sending to our internal employees.  The same sender sent me a successful email about an hour later.  I need to get this resolved ASAP as it has become a very large problem.  I have change the names of the internal employees to employee1 and employee2 and the sender to sender1 to protect the email addresses and names.

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  internal1@invent.org
    retry timeout exceeded
  internal2@invent.org
    retry timeout exceeded

------ This is a copy of the message, including all the headers. ------

Return-path: <sender1@earthlink.net>
Received: from [69.81.137.133] (helo=[192.168.1.103])
by elasmtp-banded.atl.sa.earthlink.net with esmtpa (Exim 4.67)
(envelope-from <sender1@earthlink.net>)
id 1KKIJu-0006Ck-Ry; Sat, 19 Jul 2008 15:42:19 -0400
Mime-Version: 1.0 (Apple Message framework v753.1)
To: John Doe <employee2@invent.org>
Message-Id: <889CFD9C-BF1F-4BB6-A23F-AF20E7877E38@earthlink.net>
Content-Type: multipart/alternative; boundary=Apple-Mail-12--181347540
Cc: Jan Doe <employee1@invent.org>
Subject: E-Mail Errors
From: sender1 <sender1@earthlink.net>
Date: Sat, 19 Jul 2008 15:42:17 -0400
X-Mailer: Apple Mail (2.753.1)

--Apple-Mail-12--181347540
Content-Transfer-Encoding: 7bit
Content-Type: text/plain;
charset=US-ASCII;
delsp=yes;
format=flowed