・Mailer
Thunderbird
https://www.thunderbird.net/

・Messenger Software
Signal
https://signal.org/
Element
https://element.io/
Telegram
https://telegram.org/
Discord
https://discord.com/

・Collaboration Platform Software
Microsoft Teams
https://www.microsoft.com/en-us/microsoft-teams/group-chat-software
Zoom
https://zoom.us/