Emails Bouncing

Hi,

I am facing issues with MS outlook 2007, when some user trying to send mail, it is bouncing back with following message "

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From: System Administrator
Sent: Thursday, February 21, 2013 12:56 PM
To: xxxxxx
Subject: Undeliverable: **********************

Your message did not reach some or all of the intended recipients.

      Subject:  FW: **********************

      Sent:     21/02/2013 12:56 PM

The following recipient(s) cannot be reached:

      'xxxxxx' on 21/02/2013 12:56 PM

            554 5.7.1 Message refused by Heuristic check. 9.9.9

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What might be the reason for this issue and how can we rectify this?

Thanks