Authorizing users from external servers fails?

Folks,

running a local openfire installation, I am very well able to add “local” users to my roster and have them correctly authorized. While adding users on external servers (jabber.org, dukgo.com, …) however, finding and adding the user works, but authorization does not. Either I do not see an authorization request message on “my” or “their” end at all, or accepting it (or re-sending it using the XMPP client) does not do anything. Server-to-server communication is enabled, and, even though unauthorized, message communication between the two users works well. Openfire is 3.8.1 behind a firewall, but the relevant ports should be opened.

Ideas, anyone? Any way to track down what happens so to at least produce debuggable information?

TIA and all the best,

Kristian