When the DGA (lang=fr) needed a secure email client and couldn't find a commercial one, they naturally also used FOSS and used Thunderbird to create Trustedbird.
The email client supports a number of useful features such as triple wrapping (sign, crypt, sign), signed receipts, delivery status notification...
As expected from users of FOSS, the new developments are being open sourced and could well end up in a Thunderbird near you one of these days.
And just so that it's out of the way, " (.deb,
(Disclaimer : despite my being French, I have nothing whatsoever to do with the project)"