Citing the need to build up its hosted services to attract major corporate customers, Google is acquiring communications security company Postini for $625 million in cash. Google plans to tie Postini’s security features into its Google Apps, which includes Gmail, Calendar, Talk, Docs & Spreadsheets, and Personal Start Page. In a Google blog posting explaining the reasoning behind the purchase, Dave Girouard, VP and general manager for Google Enterprise claimed that more than 1,000 small businesses sign up for the Google’s services daily, but large businesses have been reluctant to move to hosted applications due to security concerns and corporate compliance. The hope is that Postini’s services will help assuage those fears. Release..more after jump.

From an FAQ on the acquisition posted by Google (PDF link): “We agreed to acquire Postini in order to create a more complete Google Apps solution that addresses the information security and compliance issues facing businesses of all sizes.”

NYT Bits blog: “Google can be expected to have a bias toward offering its new services aimed at corporations free with advertising. It has already done that with several of its smaller acquisitions…Even if some companies would pay to offer their employees ad-free tools, Google’s global network of services may be able to help it underbid smaller rivals.”