First of all I would like to say a big "thank you" to you and your team for the excellent Enigmail codes. In particular, your assistance was a big help to my project.
I have now successfully created an extension that encrypts email using public keys encryption and password-based encryption (PBE) using Mozilla NSS that has the desired Content-Type of "multipart/encrypted" for all outgoing emails, without the use of external application.
But unfortunately, Mozilla NSS currently only has C++ implementation available and I can't get rid of C++ code for the time being. For now, I will just have to re-compile the component every 6 weeks for the time being.
Brian Teh, Singapore