Google has agreed to buy messaging security company Postini for $625m in a move to increase the appeal of Google's hosted applications among big businesses, the companies announced today.

Postini provides messaging security, archiving, policy enforcement and other services to about 35,000 business customers around the world, Google said. The vendor plans to use the technology to boost the security and compliance features of Google Apps, its hosted suite of productivity applications.

Google said around 1,000 small businesses are signing up for Google Apps every day. But it admitted that big businesses have been reluctant to use such hosted services because of concerns about security and corporate compliance. Buying Postini will help to relieve those concerns, the company said.

The cash deal is subject to customary closing conditions and expected to close by the end of the third quarter, Google said.

Google plans to make Postini a wholly owned subsidiary and said it will continue to support Postini customers and invest in Postini products.