How can I combat mail spoofing?

How can I combat mail spoofing? I work with a number of different cleints and many of them have been reporting that they are recieving NDR's for messages they did not send. It seems like this has increased dramatically over the last two months from over 7 different cleints who's small domains I manage.

I know I can block the NDR domain wide, but then they won't get an NDR if they mistype someone's name, or there is a real communication problem going on.

I have two questions.
1. Can any harm come from spammers spoofing my cleints addresses? Can an ISP block my domain if enough spam  comes from a spoofer? (I have Reverse PTR's set up, which is likley why the mail is being blocked when it's deliverd)

2. Can I block the NDR's that are returned from spoofed email, while letting NDR's through when one of my users makes a real mistake in typing

thanks for the help

I'm attaching a message that was sent to my by a cleint. He said he never sent a message to the original recipient

David

 -----Original Message-----
From:       Mail Delivery System [mailto:Mailer-Daemon@perfora.net]
Sent:      Friday, April 18, 2008 05:03 AM Pacific Standard Time
To:      Edward Smith
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 addresses
failed:

quota exceeded:
  info@HabanosyHermanos.com

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Subject: Out of Office AutoReply: H & H - The perfect present
Date: Fri, 18 Apr 2008 05:03:04 -0700
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