SMTP error 451 SBS 2003 SP2

Troubleshooting an issue with exchange server on SBS 2003 SP2,
One of there clients are trying to send through emails, they keep getting this error message back,
Checked smtp and see a current session from omr2.mail.isx.net.nz and just times out.
Any idea what might be causing this to happen?

Reporting-MTA: dns; omr2.mail.isx.net.nz
Arrival-Date: Fri, 20 Aug 2010 09:55:56 +1200 (NZST)

Final-Recipient: RFC822;
Action: delayed
Status: 4.4.2
Remote-MTA: DNS; mail.------------.com
Diagnostic-Code: SMTP; 451 admin@------ reply: read error from mail.------.com.
Last-Attempt-Date: Fri, 20 Aug 2010 14:06:14 +1200 (NZST)
Will-Retry-Until: Wed, 25 Aug 2010 09:55:56 +1200 (NZST)


MuscatechAsked:
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Paul TozerConnect With a Mentor Commented:
ACL73941 -  ACL may be Access Control List

ie some firewall/spam rule
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slappa1Commented:
if the reporting mta is not your server or your smart host the problem is with the recipients email server not your configuration.

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Paul TozerCommented:
Normally you would be delayed if the receiving email server detects you as a bulk sender. Either through their spam filter or their providers.

This can happen if you send via a dynamic ip address, or if they have misconfigured their spam filter. Get them to add you to their whitelist and see how you get on.
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MuscatechAuthor Commented:
omr2.mail.isx.net.nz 2134336901   MTA.MESSAGE.STATUS   ACL73941          ,alison.linthorne@---------.com  Deferred (from mail.----------.com [----------------])

This is the message the other external client(other email server) is getting on there end.
When trying to send to the server, I do see the other server making a connection in current sessions but does not show up in mail queue.
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MuscatechAuthor Commented:
Does anyone know what the above message could be caused by?
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