command rejected: Host not found

We receive this error message when trying to email a specific vendor. I would like to be able

The following recipient(s) cannot be reached:

      duprat@lot-oriel.fr on 29/06/2009 14:57
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            <server.ourcompanydomain #5.7.1 smtp;550 5.7.1 <server.ourcompanydomain>: Helo command rejected: Host not found>



I know I should run a dns report...i'm not sure what i'm specifically looking for in their report or in ours.
SmallPrintAsked:
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Rick FeeMessaging Engineer - Disaster Recovery EngineerCommented:
I have run into this once in the past and it might be the same thing.    The anti-spam company was looking back to the sender and was seeing something beyond what the sender had in the MX record and would deny it.   Very strict and very odd.

For Exchange 2003 the fix was:    

 1. Open Exchagne System Manager. Navigate to Administrative Groups, Site, Servers, Servername, Protocols, SMTP, Default SMTP Virtual Server,
 
 2. Highlight Default SMTP Virtual Server, right click properties.  
 
 3. Go to the Delivery Tab, and click Advanced Button.
 
 4. In the "Fully-qualified domain name: field, enter your fully qualified external domain name. Click Check DNS to verify that it resolves.
 
 5. Restart SMTP service.  

For 2007 you need to modified the receive connector for the internet incoming email.
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