https://pubmed.ncbi.nlm.nih.gov/8262106/

mu4e+mbsync+msmtp

The ancestral high-protein diet: drinking the tears of your fallen enemies.

Not yet on https://signal.org/android/apk

That could be very useful when using a text-based web browser (e.g. TUI)

I am not an expert, but I see that the Apache license has no copyleft.

I don't know about OP, but it is available in https://thecapslock.gitlab.io/fdroid-patched-apps/fdroid/repo and https://calyxos.gitlab.io/calyx-fdroid-repo/fdroid/repo

There is a payment option inside the app. It uses Stripe to process the payment.

There is also Element Call at call.element.io. I have not tried screen-sharing but it's worth a look.

Securely sharing end-to-end encrypted messages between devices is inherently difficult, especially combined with device verification. Note the 'secret chats' of Telegram are not shared across devices.

As far as I know, Matrix is the only network that has solved this problem. It even has cross-signing and federation.

obligatory xkcd

FairEmail is the best email app I've ever used.