Messaging

This is a Messaging Framework to allow message sending in a channel independent way. It will provide a common API for message composition and sending while allowing plug-ins for multiple messaging methods.

When using this framework, you won't send e-mails to users anymore. You will send them 'messages' and they will decide how they want to get them delivered, that may be by mail, IM, SMS, depending on user's preferences...
New in 6.x-4.x version (beta)
# Advanced queueing, fine tuning, drush processing.. Notifications Tools (Utility modules for sites with high load of Messages and Notifications) # This version will work with 4.x versions of Notifications, related modules tagged as 4.x, and not yet with Organic Groups
Sending methods:

Don't forget to enable and set up the sending method plug-ins you want to use, at least one is needed for it to work. Currently supported are:

* 4.x onlyApplication Toolbar (Appbar)
* New Prowl: iPhone push notifications (Drupal 6 only, requires Prowl push notifications module)
* New Twitter: developed by Will White (Drupal 6 only, requires Twitter module)
* Instant Messaging: XMPP, using XMPP Framework (Drupal 6 only, requires XMPP Framework 6.x-2.x version)
* SMS, developed by Tim Cullen (Requires SMS Framework)
* Plain text Mail: simple gateway using Drupal standard mail functions
* HTML mail: PHPMailer, contributed by Ted Serbinski
* Privatemsg, integrating with Privatemsg module (For Drupal 6 it needs the dev version of privatemsg)
* Mime Mail, by Tim Cullen (Requires Mime Mail module)

Other features:

* Suport for file attachments, with this Notifications Add-on
* Provides a sending method agnostic API for composing and sending messages
* Handles message composition and formatting depending on sending method
* Supports multiple plug-ins for different message methods
* Supports 'push' and 'pull' message delivery models
* Provides an aditional "html to nice text" filter when html_to_text is installed (Not needed for Drupal 6)

Modules using Messaging

* Notifications, a complete subscriptions framework which uses Messaging as the sending library. It is also the reference implementation of the full Messaging Framework capabilities.
* Messaging Send To, allows forwarding content through different channels (mail, sms, user defined..).

Note that this Messaging framework doesn't provide any visible functions to the user on its own, it just provides messaging services for other modules.
source: http://drupal.org/project/messaging