Oh I must've mixed up in my head Exchange support for S/MIME then. It's been a while since I had to use Thunderbird at work (using Windows recently) so I forgot the reason. I do remember someone mentioning it can only read those emails though, not really write / encrypt them.
Looks like the last comment is complaining about not being able to encrypt mail with the plugin working. I am not convinced S/MIME is fully working, or if it is, there's plenty of UX issues with it's current implementation.
S/MIME works just fine in Thunderbird, although the UI is pretty poor. I will believe that composing S/MIME doesn't work in Exquilla: Exchange uses a different format for representing messages than MIME, and the hook that it uses to build messages happens before S/MIME gets introduced, and the S/MIME code itself pretty heavily bakes in the fact that it's emitting MIME.
Mostly, yes, once you jump through the hoops of getting a personal certificate (let's encrypt style automation as plugin would be great!).
But there appears to be some issues with adding recipient certificates that are structured in a specific way. Something about the Subject format being valid for s/mime but not recognized by mozilla's cerfificate store as an other-people certificate and as a server cert instead.
This was a certificate used by a local government agency. That made it quite difficult to communicate with them securely. This is one of the few times where I really needed secure email and that's when it failed me.
The UI for adding certificates is, as I recall, a massive clusterfuck. That's due in large part to this traditionally being the domain of the certificate manager folks from Firefox, where a) the necessary components are extremely low-usage (when was the last time you used a SSL client certificate?), and b) some of those people don't give a fuck about Thunderbird.
