If you want to communicate safely with your people, whoever they are and wherever they are, you might be interested in eZing-infrastructure. All you need is a mailserver or even some mailaccounts and a php-driven webserver.
Together with eZing-infrastructure and the eZing app you are ready to run a messaging-service on your own without being bothered by anyone. To ensure there is no backdoor your get the whole code and documentation to install the app on android or iOS.
If you like the idea, please contact me at email@example.com