Encrypting Email on OS X and iOS Tutorial

My apologies if I’m posting in the wrong place. In Professor Patterson’s video: https://www.youtube.com/watch?v=XMhHjXkRLhk he describes how to use digital signatures and encryption when emailing using the Comodo email certificate service. Comodo in it’s current form no longer issues free certificates (for longer than a month) and I can’t find a suitable alternative. Most certificate authorities only offer month-long free trials and others, such as Actalis do not have the handy certificate “revocation” option. Does anyone know of service that issues S/MIME certificates like Comodo used to for free?