From phpBB Development Wiki
This might need to be split into multiple components
Generic Messaging Service
Allows sending messages / notifications via any of the available protocols. Should also allow sending with multiple protocols. This should be used whenever a user is informed about any process within phpBB. Based on the user's settings or selections the notification will be send with the prefered protocol.
Concrete Protocol Services
Ordered by priority
One could add a service that creates posts in a forum on the board. This might make sense for notifications about moderation activities.