There’s an interesting Thunderbird project going on to add some enterprise features to Thunderbird. It’s called Milimail, and it’s backed by the French Ministry of Defence and British Telecom. See www.milimail.org for more info.
Where appropriate, we hope to roll some of this work into the core Thunderbird.
For example, they’ve just implemented triple wrapping. A triple wrapped message is one that has been signed, then encrypted, then signed again. While not quite as exciting as this Tripel, it’s still exciting. See bug 380624 for more info.