554 Error Sender Address Rejected: Domain Not Found?

Hello,

Can anyone help me with this error that I'm getting?
It seems to be that the receiving email server is rejecting my reply to email address.
I have already checked that an A record and a MX record does exist, thus leads me to the conclusion that it's not our server here.
Below is the mail delivery message.

Thanks.

-Peter
Institute of Governmental Studies
510-642-7505

------------------------------------------------------------------------------
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:

  TNagel@co.lassen.ca.us
    SMTP error from remote mail server after RCPT TO:<TNagel@co.lassen.ca.us>:
    host las-gwy.co.lassen.ca.us [63.118.162.10]:
    554 <pete@cain.berkeley.edu>: Sender address rejected:
    Domain not found

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

Return-path: <pete@cain.berkeley.edu>
Received: from w9.igs.berkeley.edu ([128.32.52.69] helo=[192.168.1.100])
        by calmail-fe6.berkeley.edu with esmtpsa (TLSv1:AES256-SHA:256)
        (Exim 4.60)
        (auth plain:petefong@berkeley.edu)
        (envelope-from <pete@cain.berkeley.edu>)
        id 1FtSYL-0006el-LG
        for TNagel@co.lassen.ca.us; Thu, 22 Jun 2006 10:01:13 -0700
Message-ID: <449ACCD7.8080204@cain.berkeley.edu>
Date: Thu, 22 Jun 2006 10:01:11 -0700
From: Peter Fong <pete@cain.berkeley.edu>
User-Agent: Thunderbird 1.5.0.4 (Windows/20060516)
MIME-Version: 1.0
To: TNagel@co.lassen.ca.us
Subject: test
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-Ucb-Envelope-Recipients: TNagel@co.lassen.ca.us

test
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Received on Thu Jun 22 10:31:51 2006