GPGMail - GPGMail is an open source plugin for Apple Mail.
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
YAATA - YAATA app offers features that let you send multimedia files such as images, videos, audio, and GIFs, etc.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Easy Message - Easy Message app comes up with features to help users in effortlessly sending text messages to a phone number, which is not in the contact list.