5.0.0 smtp; 550 Administrative prohibition

I have a client that sometimes we cannot receive his emails from
it happens when he includes other email Recipients, the email usually just has a link and is addressed to no more than 25 others. I have many other clients that can send us bulk emails over 25 with links and attachments up to 15mbs and it's not a problem
We use Symantics.clould service for spam filtering, I just called thier support and they tell me the rejection is coming from our email server not their filters

the error is:
Diagnostic information for administrators:
 
Generating server: server-17.bemta-7.messagelabs.com
 
user@company.com
mail.company.com #<mail.company.com #5.0.0 smtp; 550 Administrative prohibition> #SMTP#

any suggestions where I can look?
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jsarinanaI.T. ManagerAsked:
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KimputerCommented:
If what you replace with user@company.com and mail.company.com was actually your company domain, then it's true, your server rejected the email.
Can you tell me which email server software you're using?
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jsarinanaI.T. ManagerAuthor Commented:
Exchange 2010
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jsarinanaI.T. ManagerAuthor Commented:
thanks
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