Hi,
Thanks for the interest. This is not planned for now. I'll put it down in the suggestions list, but the most desired feature enhancement at the moment is the ability to send only signed messages and add signature per message or by default. I'll be posting a dev build with the mentioned function.
The in line, according to user demand, is S/MIME for Exchange. If we get more users requesting the same thing you did, I'll make sure to give the request a higher priority.
Regards,
Nedialko