Postini will become a wholly owned subsidiary of the search giant, and its products will be used on Google's existing services.
The acquisition is particularly useful to Google as it rolls out more online applications, which in the future will also be available to offline users.
Eric Schmidt, chief executive at Google, described the move as reinforcing the company's commitment to delivering hosted applications.
"With the addition of Postini, our apps are not just simple and appealing to users, they can also streamline the complex information security mandates within these organisations," he said.
Existing Postini customers will still get full product support, and Google has stated that it has no plans to change any current product lines.
"As the market leader in on-demand secure communications and compliance solutions, Postini complements Google 'perfectly'," said Quentin Gallivan, president of Postini.
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