Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
MailEnable - MailEnable is a Windows based mail server and email server program that is packed with the state of the features and functions like antivirus and spam filtering system, webmail management system, cardDAV, CalDAV and much more.
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.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.