An inbound mail gateway is a server through which all incoming mail for your domain passes. The Postini servers are an example of an inbound mail gateway: they filter (and possibly archive) incoming mail before passing it on to the Google Apps mail servers. You'll want populate Google Apps inbound gateway with Postini server IP's.
|System||IP Range||CIDR Range||IP/Subnet Mask Pair|
|5, 6, 7, 8, 20||22.214.171.124 - 126.96.36.199||188.8.131.52/20||184.108.40.206
|9||220.127.116.11 - 18.104.22.168||22.214.171.124/22||126.96.36.199
|10||188.8.131.52 - 184.108.40.206||220.127.116.11/22||
|11, 20, 200, 201||18.104.22.168 - 22.214.171.124||126.96.36.199/20||188.8.131.52
How do I know which system we are on? -
Take a look at the URL after signing into the Administration Console, it is indicated in this screenshot , I'm on system 7.