PARIS, USA: A new email client unveiled by Mozilla this week contains code from an unusual source -- the French military, which decided the open source product was more secure than Microsoft's rival Outlook.

The story of how the French government became involved with the open source movement, which has transformed much software around the world from proprietary to free, goes back six years.

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France's military chose open source software after an internal government debate that began in 2003 and culminated in a November 6, 2007, directive requiring state agencies "Seek maximum technological and commercial independence."

The military found Mozilla's open source design permitted France to build security extensions, while Microsoft's secret, proprietary software allowed no tinkering.

"We started with a military project, but quickly generalized it," said Lieutenant-Colonel Frederic Suel of the Ministry of Defense and one of those in charge of the project.

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The Gendarmerie Nationale police, which was part of the military at the time and did the design, released some of its work to the public under the name "TrustedBird," and co-branded it with Mozilla.

The military uses Mozilla's Thunderbird mail software and in some cases the Trustedbird extension on 80,000 computers and it has spread to the ministries of Finance, Interior and Culture.

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The French government is beginning to move to other open source software, including Linux instead of Windows and OpenOffice instead of Microsoft Office.

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The non-profit groups that make most such programs, including Samba server programs and Mozilla's best-known product, the Firefox Web browser, depend on volunteer programmers around the world.

Thunderbird 3 used some of the code from TrustedBird, as well as drawing on 1,000 other computer geeks and users around the world.

"The primary changes (the military) have made allow them to know for sure when messages have been read, which is critical in a command-and-control organization," said David Ascher, chief executive of Mozilla Messaging.

That qualifies it for NATO's closed messaging system, and the French military has shown TrustedBird to NATO, military officials said.

The French military is "helping build an ecosystem of specialists around the world that provide specialized add-ons, leveraging our platform to help meet customer needs," Mozilla's Ascher said.

The result is a product that is compatible with Yahoo! mail, Google Gmail and other email systems, but competes against Microsoft Outlook.

Some observers said the original decision to go to open source was in part to show the French public that the government was trying to save money. But many software experts say free software also has its costs and the French acknowledged that.

"It is never completely free," said Col. Bruno Poirier-Coutansais of the information technology team Gendarmerie Nationale.

New software requires the re-training of information technology specialists. Experts point out that it also requires employees to learn to use the software.

Poirier-Coutansais said there was initial reluctance to embrace Thunderbird "because it is less reassuring than to have a renowned company that can bring quality support."

Microsoft has platoons of technicians in France at a state-of-the-art glass building in a western suburban business district emblazoned with the company logo, while Mozilla, with 10 employees in an anonymous concrete colored residential building, is distant from Paris's business districts.

Nonetheless, the French government is now pleased with the results. Suel said they may move beyond government, because Trusted Bird's security extensions make it "transposable to large international companies."

One expert was less certain of that.

"The professional market is showing more resistance to open source software," said Bernard-Louis Roques, chief executive of Truffle Capital IT, an investment fund specializing in software.