This article explains how to use MPP along with Postfix (or qmail or anything) to archive your incoming, outgoing and internal Google Apps mail domains.  There are many reasons to use MPP with your Google Apps email including our advanced reporting, economical price, avoidance of vendor lock-in and flexibility beyond what Google can offer.  The MPP Virtual Appliance can be used for this application or you can build your own MPP/Postfix server. This article will not cover the Archival setup, but only additional information on how to setup Postfix gateway.

http://www.google.com/support/a/bin/answer.py?hl=en&answer=60730. 

Inbound Gateway

For Postfix we'll follow the instructions for Inbound Gateway referenced above except that domain relaying for mydomain.com will be set from MPP Manager 

http://mpp_host:20001 -> System -> SMTP Relay -> Add new domain using Domain: mydomain.com  Host: aspmx.l.google.com and Check MX record NOT checked.

Note: Please set MX record to MPP host (hostname, NOT IP) and also Inbound Gateway setting in Google Apps panel as described in article http://www.google.com/support/a/bin/answer.py?hl=en&answer=60730.

 Outbound Gateway

The Outbound Gateway should allow relaying if mail from Google Apps would be sent through MPP/Postfix host.  To identify IP's that should be allowed to relay please use from Command Line/Terminal:
dig txt _spf.google.com | grep spf
;_spf.google.com.        IN    TXT
_spf.google.com.    300    IN    TXT    "v=spf1 ip4:216.239.32.0/19 ip4:64.233.160.0/19 ip4:66.249.80.0/20 ip4:72.14.192.0/18 ip4:209.85.128.0/17 ip4:66.102.0.0/20 ip4:74.125.0.0/16 ip4:64.18.0.0/20 ip4:207.126.144.0/20 ?all"

We'll obtain a list of network classes / IP's that should permitted to relay. Please compile a list without "ip4:" prefix: "216.239.32.0/19 64.233.160.0/19 .." and go to MPP Manager (http://mpp_host:20001) -> System -> SMTP Relay -> Advanced -> Show and add the list in IP Networks to Relay text field.

Note: Please set the Outbound Gateway setting in Google Apps panel as described in article http://www.google.com/support/a/bin/answer.py?hl=en&answer=60730.