Little communication software out there can be made as secure and resilient as email.
Two people out there can run their mail servers on a laptop or a raspberry pi. Email is easily encrypted with TLS and GPG on top.
The initial setup for this could take less than 2 hours.
Bonus points: everything could be run from a *sh shell, 1 CPU core and 128 RAM is quite enough.
> bad tooling and software
Postfix (and HAProxy for that matter) are some of the best pieces of software I have the pleasure to work with. Very good docs, I personally haven't seen them crash and have a way to never take you by surprise.