[Nolug] Mail Server Configuration Issue

This is for a friend of a friend, and so I am just looking for a general
direction to point him in.

He is sending mail through his server, which is running exim according to
the headers. It seems to be rewriting the sender to nobody@host.xxx.com,
instead of sales@xxx.com. This is causing the DNS lookups to check
host.xxx.com which isnt regestered in the DNS.

Should he just put host.xxx.com into the dns? Or is there an easy way for
him to configure his mail server to actually say its from sales@xxx.com.
Again, sorry for the lack of information, but this is just what I could get
over the phone and via email.

Attached bounced mail:

-----Original Message-----
From: Mail Delivery System [mailto:Mailer-Daemon@host.xxxx.com]
Sent: Sunday, July 05, 2009 12:02 PM
To: nobody@host.xxxx.com
Subject: Mail delivery failed: returning message to sender

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:

 recip_name@recip_domain.com
   SMTP error from remote mail server after RCPT
TO:<recip_name@recip_domain.com>:
   host mail.recip_domain.com [64.26.60.153]: 450
<nobody@host.xxxx.com>:
   Sender address rejected: Domain not found: retry timeout exceeded

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

Return-path: <nobody@host.xxxx.com>
Received: from nobody by host.xxxx.com with local (Exim 4.69)
       (envelope-from <nobody@host.xxxx.com>)
       id 1MLjId-0001hO-Gl
       for recip_name@recip_domain.com; Tue, 30 Jun 2009 15:47:27 -0400
To: recip_name@recip_domain.com
Subject: Your XXX, Inc. Invoice No.: YYYY
MIME-Version: 1.0
From: sales@xxx.com<sales@xxxx.com>
Cc:
Content-Type: multipart/alternative;
       boundary="=_968a779a98858486b4cf786479455aa2"
Message-ID: <km2hn3.sd8zm8@www.xxxx.com>
Date: Tue, 30 Jun 2009 15:47:27 -0400

--=_968a779a98858486b4cf786479455aa2
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7bit

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Received on 08/21/09