A PMS usually allows to send internal messages only.
In uddeIM 1.5 there was a pre-release of a "Public Frontend" for testing purpose implemented. From uddeIM 1.6 on this has become a "Premium Plugin" since this is not a basic functionality of an PMS.
You find more infos about the Premium Plugins on my dev site.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer