Allow external xmpp clients in my group chatroom

I would like to allow users from other XMPP servers access to my group chatrooms.

Currently it’s not working, but I can’t find where to enable it.

Direct chat from external servers (I used google talk to test) to my local users works perfectly.

Besides Google talk, I tried serveral other xmpp providers from this list:

I noticed that none of the public providers in the list above allow the users on my own server into their chatrooms.

Is this impossible to accomplish using Openfire?

So far, nobody replied to my question. Is the question not clear enough? Anything I can do to help?