[Mailman-Users] Why are we seeing these errors?

[Gustine, Charity G.]

We just recently installed Mailman 2.1 on our Unix server. We have gone
over the install document several times to see what we missed. We would
like our users to be able to send an email to the list with subject
subscribe. When we test this we never see the mail messages.
It would appear that it is a domain issue. Here is what we see in the
logs when we try to do this: 
-----Original Message----- 
From: "Postmaster" [mailto:"Postmaster" <mailto:> ] 
Sent: Tuesday, May 24, 2005 10:20 AM 
To: "Undisclosed Recipient" 
Subject: Delivery Notification
<ztmp-request at bthnsoweb01.wnsnet.attws.com> 
This is a delivery status notification, automatically generated by MTA
extmail12.cingular.com on Tue, 24 May 2005 13:20:21 -0400 Regarding
recipient(s) : ztmp-request at bthnsoweb01.wnsnet.attws.com
Delivery status : Failed. Could not send the message. Domain address
<bthnsoweb01.wnsnet.attws.com> is not valid. Please consult your domain
administrator to check the domain before sending the message again.
Message Information: 
Sender : ron.wislen at cingular.com 
Recipient : ztmp-request at bthnsoweb01.wnsnet.attws.com 
MTA Response :510 
The original message headers are included as attachment. 
We are able to get confirmation, via email, when we create a new mailing
list. 
When we subscribe to a list via the web site we successfully get
subscribe and a confirmation email is sent back to the user, after about
10 minutes. 
We are seeing this message in /usr/local/mailman/logs 
May 24 08:00:03 2005 (16988) delivery to
ron-test-mailman-owner at servername.com failed with code 550: 5.1.1
<ron-test-mailman-owner at servername.com>... User unknown.
Any ideas?