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.