Signal
Signal is a free, privacy-focused encrypted messaging app from the Signal Foundation. Signal Desktop 8.28.0 brings your chats to a Windows PC.
Windows
Open source
Open source: Communication
Open source programs in the “Communication” section: Messengers, Email clients.
Messengers
Instead of WhatsApp Desktop, WeChat, WhatsApp, QQ
Email clients
Instead of Microsoft Outlook, Mailbird
Thunderbird
Thunderbird is a free, open-source email client and personal information manager for Windows, macOS, Linux and Android.
Windows
Open source