I need to run a, secure, e2e instant messaging server for a small group of people (< 10).

The requirements are:

* iOS / Android client with push notifications
* Linux & BSD clients
* on-prem private server

What do people recommend?

[Edited to explain I want to run it on-prem for a closed group of people - sysadmins actually on a delicate project]

@cynicalsecurity first thing that came to mind is @delta with an own relay, but I'm unsure about the BSD clients ...

https://delta.chat/en/

Delta Chat: Delta Chat, decentralized secure messenger

Delta Chat is a decentralized and secure messenger app 💬 Reliable instant messaging with multi-profile and multi-device support ⚡️ Sign up to secure and interoperable chatmail relays 🥳 Interactive ...