Thunderbird Android supports email encryption and signing using OpenPGP. I would like to add support for S/MIME. Our company Ciphermail (www.ciphermail.com) provides an open source email encryption server product. We also have Ciphermail Android, a free app for encrypting and signing email. Our proposal is to convert this app to a companion app to Thunderbird, as a plugin, in much the same way that Thunderbird Android now supports OpenPGP. We would make our app open source.