Exchange 2007 rejecting messages

I have an exchange 2007 installed with content filter activated and also the antispam feature, and sometimes i have user saying that there are complains from people that are trying to send email to them that they get the following message:

Reason: SMTP transmission failure has occurred
Diagnostic code: smtp;554 5.6.0 Invalid content
Remote system.........Microsoft ESMTP MAIL Service ready at Wed, 16 Jan 2013 15:39:26 +0100)

Any ideas on how to solve this issue?
WindindiAsked:
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Suliman Abu KharroubIT Consultant Commented:
Check SMTP on http://mxtoolbox.com
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