01.net today reported (French | Google Translate) that two governmental institutes that recently merged will be moving to a unified Open Source-based solution. The institutes is the General Tax Directorate (DGI) and the Directorate General of Public Accountancy (DGCP) merged to create the General Directorate of Public Finance (DGFiP).
DGI with 80,000 users previously used Lotus Notes, while DGCP with 50,000 users was using Microsoft Outlook (presumably with Exchange, but the article does not reveal this). On the desktop-side, this new solution will be using Mozilla Thunderbird for email with the Lightening calendar extension.
It might also be worth mentioning that this is not the first time a French government switches over to Thunderbird. The Department of Defence for instance, have already switched over 70,000 users over to Thunderbird.