GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
CrococryptFile - Strong file encryption tool
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
VSEncryptor - VSEncryptor is an easy to use file and text encryption program.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.