After the launch of Google Apps Premier Edition, more than 100,000 companies switched to Google's hosted services, but large companies couldn't use it "due to issues of security and corporate compliance". To provide better services for these companies, Google bought Postini, "the global leader in on-demand solutions that deliver on the promise of communication security and compliance". Postini is also a variation of postino, the Italian word for postman. Like Google Apps, Postini doesn't require you to install special software or hardware, as all the data is hosted and processed on their servers. "Postini invented the software as a service approach to providing communications security and compliance, and holds two fundamental patents in the space, with more patents pending."
Postini lets you recover messages, set policies for individual users or groups (block all the messages that contain a certain a keyword), archive and delete messages according to company's policy. "When an email message is sent, it passes through Postini's datacenters and a copy is saved while the original continues on its way. All of this happens within milliseconds, so there is no delay to your messages. When you configure rules for your users we can also check for violations and, if necessary, block the message or take other actions that you define."
Postini has already been a Google Enterprise Partner for Google Apps and it serves 35,000 business clients. The press release notes that Google paid $625 million in cash.