Exchange wont send email to particular domains

For the most part, our email works great.  However we are getting some failed emails to some (two) outside organizations.  The error in the Exchange Queue Viewer is Error: 450 4.7.1 Client host rejected: cannot find your hostname, [IPADDRESS].  The issue I see is the IP address it lists is the ip address we use to browse the internet, not the SMTP address.  There are reverse DNS and pointer records for both our primary and secondary MX records.
SidwellITAsked:
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Simon Butler (Sembee)ConsultantCommented:
Sounds like you haven't got your outbound NAT configured correctly on the firewall.
You need to check there so the traffic is going out the same way as it is coming in.

I would do that rather than trying to get the traffic accepted on your default IP address, that way you reduce the risk of getting blacklisted.

Simon.
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s3e3Commented:
It sounds like your firewall issue.
Are you sure your firewall is passing outbound SMTP traffic using the the IP address of the SMTP server ?

You can double check by sending yourself an email and looking at the headers.
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R--RCommented:
It should be the outside IP of the firewall.  Please try to telnet to the recipient mx from your network using telnet and check what output u get.
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SidwellITAuthor Commented:
When I view the header of an email I sent to my gmail, it is from the outside IP address of the firewall,

Received: from smtp-gateway.domain.edu ([OutsideIPofthFirewall])
        by mx.google.com with ESMTPS id 4si2386828qat.114.2014.03.05.19.36.39

I would have thought that it would use the public DNS entry for smtp-gateway.domain.edu instead of the IP address of the firewall.
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s3e3Commented:
no, the dns entry and firewall are interdependent.
you can either fix this from the firewall or adjust the dns servers and ensure the outbound ip has a reverse dns entry for sending mail. Maybe option 1 is easier depending on the firewall make and model .
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SidwellITAuthor Commented:
Thanks guys!  Changing the Source IP for outgoing SMTP traffic solved our problem.
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