Secure chat system for remote communication/Free version provided
nsoft-s.com
nsoft-s.com
Its completely open source and very easy to setup imo. And, the entire French government trusts it and uses it for day to day work, so its not going to be abandoned any time soon.
https://matrix.org/blog/2018/04/26/matrix-and-riot-confirmed...
If you want UI polish, Zulip is still ahead. If you want federation and interoperability, Matrix is a strong contender.
The only thing remotely related to security on that page is that you use TLS in a client/server setting. That's not remotely better than what the person you're replying to suggested, which is also 100 times less suspicious looking than a random landing page with generic looking icons.