Returned Mail: delivery problems encountered

We have a client who can receive e-mails from our company, but when she tries to reply to the e-mails, she gets the following message:

The following addresses had delivery problems:

Permanent Failure: Other address status

She can email us and we can email her, she only gets the error when she replies to ANY of our emails.

We are running Exchange 2003 SP 2 on a Windows 2000 server.

I finally got a copy of the returned mail. For confidentiality reasons, some of the info has been change. This is how it looks like:

Received: from rwcrmhc14.comcast.net (localhost[127.0.0.1])
by comcast.net (rwcrmhc14) with ESMTP
id <20060328201212m1400rqt01e>; Tue, 28 Mar 2006 20:12:12 +0000
From: Mail Delivery Subsystem
Subject: Returned mail: delivery problems encountered
Message-Id: <20060328201212m1400o8v00e@A@000000000000@comcast.net>
Date: 28 Mar 2006 20:12:12 +0000
To:
Mime-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status; boundary=”_4429989c.611f.0+comcast.net=_”

–_4429989c.611f.0+comcast.net=_
Content-Type: text/plain

A message (from ) was received at 28 Mar 2006 20:12:09 +0000.

The following addresses had delivery problems:

Permanent Failure: Other address status
Delivery last attempted at Tue, 28 Mar 2006 20:12:10 -0000

–_4429989c.611f.0+comcast.net=_
Content-Type: message/delivery-status

Reporting-MTA: dns; comcast.net
Arrival-Date: 28 Mar 2006 20:12:09 +0000

Final-Recipient: rfc822;
Action: failed
Status: 5.1.0 MAIL FROM: 552 REPLY: 552_Requested_mail_action_aborted:_exceeded_storage_allocation
Diagnostic-Code: smtp; Permanent Failure: Other address status
Last-Attempt-Date: Tue, 28 Mar 2006 20:12:10 -0000

–_4429989c.611f.0+comcast.net=_
Content-Type: message/rfc822