Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
LAN Messenger - LAN Messenger is a decentralized UDP IM-style chat client which supports usernames...
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
MyChat - Secure instant messenger for any network
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.
BORGChat - BORGChat is a LAN messaging software program.