Delayed Mail (still being retried)

I am getting some error notification like this Delayed Mail (still being retried)from my smtp brightmail server.

****@safehandshealthsystem.com mycompany@mailid.com Monday, Jul 06, 2009 02:20:24 PM AST 451 4.3.5: Client host rejected: Server configuration error
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delivery report like this :

Final-Recipient: rfc822; *****@—–.com
Status: 4.0.0
Action: delayed
Last-Attempt-Date: Sun, 05 Jul 2009 21:05:08 +0300
Diagnostic-Code: smtp; 451 4.3.5