'Google doesn't post a public list of IP addresses for webmasters to whitelist. This is because these IP address ranges can change, causing problems for any webmasters who have hard-coded them, so you must run a DNS lookup as described next.'
If anyone can tell me how to embed this particular DNS query into a firewall appliance ACL, please let me know. I have customers with Watchguard and Cisco gear who have intermittent trouble with Google's mail relay (formerly Postini). The solution is always to perform that record lookup and add the new IP address blocs.
