I'm using the S/MIME plug-in for Airmail on my MAC (the PGP plug-in causes a crash) but it doesn't seem to be available for the iOS version. This is a significant weakness as it forces me to use Apple's default Mail app for an encrypted messages. An even better option though would be to incorporate a Virtru like encryption functionality. In other words, rather relying on an informed user to install their own certificates (and the public keys of counterparts), build it into the system such that Airmail users can send encrypted messages to each other at the push of a button and to recipients not using Airmail subject to their opening the message via an Airmail-provided web portal (which will encourage them to install the Airmail app). Note that it is important that this encryption be end-to-end and include attachments.