RRAS NAT problem with multiple external IP's

How do you tell RRAS which external IP to use for internal servers?

I have a Windows 2003 router with two NICs, one external and one
internal. I have about 10 external IPs and one internal. Inside the
network I have our exchange server that uses the router as default
gateway.

There is a port mapping on the router sending all SMTP traffic on a
specific IP to the exchange server, however when the exchange server
sends mail they get routed from another IP! The emails gets sent from
"wrong" ip and is bounced by some recipients because they don't
consider the IP valid.

I have no clue why the router chooses the IP it chooses, it's just one
in the middle of the range...

How can I control which external IP the router should choose when
sending the SMTP packets?

We need more details to help. Do you mean all 10 external IPs are assign to external NIC?

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com
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How do you tell RRAS which external IP to use for internal servers?

I have a Windows 2003 router with two NICs, one external and one
internal. I have about 10 external IPs and one internal. Inside the
network I have our exchange server that uses the router as default
gateway.

There is a port mapping on the router sending all SMTP traffic on a
specific IP to the exchange server, however when the exchange server
sends mail they get routed from another IP! The emails gets sent from
"wrong" ip and is bounced by some recipients because they don't
consider the IP valid.

I have no clue why the router chooses the IP it chooses, it's just one
in the middle of the range...

How can I control which external IP the router should choose when
sending the SMTP packets?