Google, which as we all know (wink, wink, nod, nod) isn't competing with Microsoft, is acquiring privately held Postini for around $625 million cash.

The deal just coincidently happens to strengthen its applications push into the enterprise, which may explain why Google is paying more for it - less whatever Postini's got in the bank - than it has for anything else except YouTube and DoubleClick, if the government doesn't stop the DoubleClick deal.

Postini provides patented on-demand security and compliance services for e-mail and instant messages, which should fit divinely with Google's enterprise-bent hosted applications kit and give Google a built-in reservoir of the 35,000 companies or 10 million users that Postini services to go trolling in for new, paying, Google Apps Premier Edition users.

Google says there's little overlap in the customer base. Postini counts Merrill Lynch, Invesco, LSI Logic, Johnson Controls, Mitsubishi and United Technologies as clients.

The Premier Apps package of word processing, spreadsheets, calendaring, instant messaging and e-mail, runs $50 a seat a year. Google says it got 100,000 takers after the thing debuted in February.

Credit Suisse gives the eight-year-old Postini, which has reportedly been profitable since 2004, 8%-9% of the e-mail security boundary market. Its products include message archiving and encryption, filtering and policy-enforcement.

According to a canned statement from Google CEO Eric Schmidt, "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 organizations."

Google brags that 1,000 small businesses are signing up a day to use Google Apps, but large concerns "have been reluctant to move to hosted applications due to issues of security and corporate compliance." Postini solves the problem and puts Google right in Microsoft's prized territory.

The deal, which shouldn't thrill Symantec or McAfee much either - both lose if software goes SaaS - is supposed to close by the end of September when Postini and its 300 people will become a Google subsidiary.

In the small, Postini is competing against Microsoft's FrontBridge acquisition, Symantec's Brightmail acquisition, Tumbleweed's Corvigio acquisition and Secure Computing's CipherTrust acquisition, but Google is out after bigger bear - Microsoft's 450 million desktops.

Postini will continue to sell its wares - which, as it happens, support Microsoft Exchange and Lotus Domino - independent of Google Apps. Google could offer a free, advertising-based version of its stuff as well as a subscription service.

Jefferies and Company figures Postini sales could be around $90 million this year.

Postini was funded by Sun, August Capital, Bessemer Venture Partners, Paficia Fund, Summit Accelerator Partners and Mobius Venture Capital.

Meanwhile, besides Postini, Google also just bought two-year-old GrandCentral Communications, a clever service start-up that gives you one phone number - "for life," it says - that rings your office phone, home phone and cell phone simultaneously for an undisclosed amount of money.

Oh, and there's a single voice mailbox too accessible online or over the phone. Doubtless this is somehow going to wend its way into Google Apps too.

The blogosphere put the acquisition price at around $50 million.

Google is continuing the GrandCentral beta test.