is an E2EE chat client that uses SMTP as underlying transport. Available on Linux, Android, MacOS, iOS, Wind.
Excellent and polished.
Send text, files, photos, images, voice-memos (and is possible to add on videocalls or location-sharing)
--//--
The email backend can be setup with a free account here:
https://nine.testrun.org/info.html
Or by using your own email server.
--//--
Can also install JitsiMeet as the integrated video-chat client.
ArcaneChat is an alternative/expanded frontend client.
* Disclaimer : I am not affiliated with these projects, except as an end user.