2
Here is a wiki article outlining the usage of the XoopsMailer class.
http://dev.xoops.org/modules/phpwiki/index.php/XoopsMailer
In addition, XOOPS provides mechanisms for users to register themselves to receive notifications when events happen in the system. This is harder to configure than using the XoopsMailer directly, but adds the ability to opt-in/opt-out of recieving the messages.
http://dev.xoops.org/modules/phpwiki/index.php/Notify