Here are descriptions of the more relevant changes in 16.0
End-to-end security is supported for ZRTP calls. This major version adds end-to-end security also to secure messages.
Every client has an asymmetric keypair used to encrypt/decrypt messages via NaCl box model. is the keyserver providing public keys to clients.
Read notification
Users can receive a read-notification, knowing whether and when their secure message has been read. Feature can be enable in application settings or via provisioning profile by system administrator
changes
This version carries a User Interface redesign of clients and server, plus many new devices and OS version support.
Version 16.0 brings a brand new core OS platform: legacy OS core based on Fedora Core has been replaced by CentOS 7, with a stable, robust, reliable distribution.
now supports HA cluster in Active/Passive configuration, to gain more reliability and lower downtime due to failures or maintenance.
System administrator has a clear view of licenses, including single client and temporary one used to manage transient users.
Web administration console layout has been revised to improve usability, splitting daily maintenance tasks from extraordinary tasks required rarely or during system setup.