Cisco Spam Quick Start Manual page 18

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Appendix
About Network Address Translation (for Step 10)
NAT is the translation of an IP address used within one network to a
different IP address used in another network. For example, you might want
route email to a public IP address, while keeping all of your other addresses
private. If you use Network Address Translation on your router or firewall, you
may not need to change your MX records, but you may need to configure
port forwarding to ensure email gets routed to the Blocker.
For instructions on changing your NAT translation tables, consult the
documentation for your router or firewall.
In this example, the router/firewall uses NAT to route email from the
public IP address of 192.0.2.1 to the internal IP address of the mail server
at 192.168.0.7. The
MX records do not
Firewall or Router with NAT
need to be modified,
public IP : 192.0.2.1
but port forwarding
must be changed to
route Port 25 traffic
PORT FORWARDING
to the Blocker.
Cisco Spam & Virus Blocker
private IP : 192.168.0.8
mail.mydomain.com
ADD SMTP ROUTES
Mail Server
private IP : 192.168.0.7
Before
A Record: mail.mydomain.com IN A 192.0.2.1
MX Record: mydomain.com IN MX mail.mydomain.com
Port forwarding: Port 25 traffic to 192.168.0.7
After
A Record: mail.mydomain.com IN A 192.0.2.1
MX Record: mydomain.com IN MX mail.mydomain.com
Port forwarding: Port 25 traffic to 192.168.0.8
SMTP route between Blocker and mail server
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