@craigmaloney the issue is not the application, e.g. there are many email apps, but using a standardised, and federated, protocol.

Matrix (Element) seems the best available, for security: https://privacytools.io/software/real-time-communication/#im

They also have a big focus on bridges to lots of other platforms, which helps.

i.e. bridge from one platform on one server, through to Matrix, a federated system, then out some completely different server bridged somewhere else.