The Addon gives users a mailbox for user-to-user and system-to-user messages. Such internal communication can be preferable over email in certain cases, e.g. for system messages, messages related to certain entities in the system, or requiring audit or response. A logged in user will see a menu entry called "Messages" which will show the incoming messages. There are two options to send a message: manually from one user to another or programmatically.
The addon comes with two default options for the main screen with horizontal and vertical menus. For more information, please refer to README on GitHub.