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Xoops 2 has an integrated Notification system that will perform this task. In the Forum Preferences set the "Enable Notification" option to "Enable Notification (both styles)"
Then in "Enable Events" make sure all events you want users to be able to subscribe to are highlighted.
Now when users browse your forums a "Notification Options" section will be added to the bottom which will let a user subscribe to these events, and be notified automatically.
Please note that if you have a high post count, or high user count, the amount of emails generated can overload a server. You should especially be careful in a shared hosting environment.
