It’s in beta, but it’s already quite usable. You can try it without account on a public instance e.g. at call.element.io

mu4e+mbsync+msmtp

The Malazan Book of the Fallen, by Steven Erikson.

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.

Have you heard of Beeper?

I think Matrix has bridges to all these networks, except ActivityPub (it may, I just don’t know).

Matrix is in the process of being adopted as the MIMI standard by the IETF. At least it was one or two months ago.

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

obligatory xkcd

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