The following are the current Postini outgoing IP address ranges as of November 2010 :
64.18.0.0 - 64.18.15.255 -- 64.18.0.0/20 -- 64.18.0.0 mask 255.255.240.0
74.125.148.0 - 74.125.151.255 -- 74.125.148.0/22 -- 74.125.148.0 mask 255.255.252.0
74.125.244.0 - 74.125.247.255 -- 74.125.244.0/22 -- 74.125.244.0 mask 255.255.252.0
207.126.144.0 - 207.126.159.255 -- 207.126.144.0/20 -- 207.126.144.0 mask 255.255.240.0
The range used to relay email to your server is likely only one of these ranges.
The current list can be found at the Postini support site
here.