Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
GPG Suite - Use GPG Suite to encrypt, decrypt, sign and verify files or messages.
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
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.