Users from other Jabber servers cannot use external and internal components such as PyMSNt, MU-Conference, Publish-Subscribe and vJUD. Although, everything works for users registered on my server.
Jabber/XMPP servers needs to be able to reach components directly. This involves that all component subdomains must be resolvable by the other servers.
You need to make sure that all the subdomains (conference.example.com, pubsub.example.com ...) are resolvable from the third-party servers on the Internet.
To accomplish this, you can:
Add a wildcard in your DNS configuration so that *.example.com always points to your server.
Add an entry pointing to your server's IP for all subdomains in your DNS configuration.