DALLAS (Bloomberg) — Google Inc., owner of the most popular internet search engine, agreed to buy closely held Postini Inc. for $625 million, adding e-mail security services to its programs for small businesses.Postini, based in San Carlos, California, manages data centres that process and protect customers' e-mails and instant messages. Its products have more than 11 million users at 35,000 firms, the companies said yesterday in a statement.

The Postini security features may attract more companies to Mountain View, California-based Google's Gmail e-mail service. Google will rival Microsoft Corp.'s Exchange and Outlook software applications by encouraging small businesses to adopt Gmail along with its calendars, spreadsheets and instant messaging.

"This brings Gmail up to enterprise class, embedding the security," said Peter Firstbrook, a research director at Stamford, Connecticut-based Gartner Inc. "They need to compete directly with what Microsoft is offering."

Postini is one of the three largest e-mail security service companies, along with MessageLabs Ltd. and Microsoft's Exchange Hosted Services, Firstbrook said. Each company has about $50 million to $60 million in annual sales, he said.

Google, which controls more than half the US internet search market, is entering new businesses such as web-based applications and mobile software as sales growth slows. Chief executive officer Eric Schmidt has orchestrated more than 10 acquisitions in 2007.

The Postini purchase, expected to close by the end of September, is Google's second-biggest this year and third overall. The company agreed to buy DoubleClick Inc. in April for $3.1 billion and completed a $1.65 billion purchase of YouTube Inc. in November.