@rramsey
Look at that part of my question to krileon and his answer,

OK, have uploaded on a live server, for testing, using php mail instead of smtp (as set in my localhost) and works perfect. If, as you said, GJ is using the exact same function as CB Invites, why is GJ Invites working on my localhost and not CB Invites?

Don't know, I've made a note to rewrite the notification function for next release using GJs source.

As your server is not accepting php mail, seems that you will have to wait for the next release

Well ... I'll have to wait I guess ... pity ... nice extension!