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Outgoing SMTP connection using different IP that incoming IP. Causing reverse DNS issues

I am currently running a 2003 Exchange Standard server. Router IP is 1.2.3.4 incoming smtp is 1.2.3.5. Reverse DNS is setup correctly to 5.3.2.1 yet when the server makes an outgoing connection it is coming from 1.2.3.4 and being denied for rDNS due to the incorrect IP. How do I make Exchange use the correct outgoing IP for this to work correctly? Or do I have to change my NAT to move the Exchange server to the Routers IP?
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Felix Grushevsky
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So the reverse dns should point to the router and not the IP used for the mail server? Crap!
What do you have in front of the Exchange server? When you go to whatismyip.com from the Exchange server, what do you get as an IP address? If it is something other than 1.2.3.5, then you have a routing problem with your firewall/router.
Reverse DNS should be pointing to whatever is the external IP address of the Exchange server.