Thunderbird

" 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.

Thunderbird gained 130,000 users as part of a merger of two French tax authorities: The Directorate General Tax (DGI) and the Directorate General of Public Accountancy (DGCP). They have switched to Thunderbird because they didn't want to renew the licenses of the old proprietary software which were Lotus Notes and Microsoft Outlook.