Seald - Keep your conversations secure, everywhere.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Crypho - End-to-end-encrypted messaging and file sharing
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
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.