Google Outbound Services Configuration Guide - Manual (Page 20)

Introduction to Outbound Configuration 21
Publishing an SPF record following the format described by the SPF wizard
should not affect inbound mail flow.
Alternate Option: Routing Outbound Mail on Your Firewall
Most of this guide contains instructions for how to set up your mail server to route
mail to Outbound services. Another option for routing involves setting up Postini
outbound using Network address translation (NAT) and stealth-proxying port 25.
This requires intimate knowledge of your firewall configuration, and is different for
each firewall. If you are familiar with firewall implementation, this can be a simple,
flexible way to configure Outbound Services, without the complexity and possible
performance problems of a smarthost.
To do this, set your firewall to NAT outbound (i.e. non-local) TCP port 25 traffic to
be from your defined external gateway IP address, and to the Postini Outbound
load-balancer that is appropriate for your Postini system.
Also, use NAT to configure reinjection of mails back to your mail server. You can
do this by relabelling TCP port 25 packets from the Postini IP range so they
appear to come from inside your LAN. This is called Port forwarding and is a
common configuration option for many IP firewalls.