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
Sends a regular private message on the board.
Sends an e-mail to the recipient's address.
Sends a message via XMPP. Are there good external XMPP libraries? Shall we write our own (and contribute it to a framework)?
One could add a service that creates posts in a forum on the board. This might make sense for notifications about moderation activities.
Implementation of IRC for notifications of specified events to a channel for things such as moderation notices or announcements, or as a private message for things like subscription notifications (like XMPP).
Possibly have a list of IRC accounts setup by administrators, then when setting up a notification event the user can select the account that will be used to notify them (allowing for different servers or login requirements for IRC accounts).