Well, from the earlier comments, it sounds like any mailserver with Postini in front of it produces headers much like:
Received: from psmtp.com ([188.8.131.52])
by alias.acm.org (ACM Email Forwarding Service) with SMTP id COB73880
for <argus[at]acm.org>; Sat, 08 May 2004 01:13:01 -0400
Received: from source ([184.108.40.206]) by exprod7mx45.postini.com ([220.127.116.11]) with SMTP;
Fri, 07 May 2004 22:13:00 MST
The question is whether adding code in the parser to recognize such a pair of Received lines would be easier than doing a separate waiver for each of Postini's customers. Note that the 18.104.22.168 MX host does seem to be shared with arkansas.net and presumably others, so the Mailhosts code could theoretically recognize a Postini customer before it even sends the mail.