Is it possible to post the XML being sent from client to server? Psi should work.
RhymBox has some known problems connecting to Messenger because it hangs waiting for a response to the optional iq-browse protocol (Messenger properly generates a ‘‘not supported’’ response but RhymBox ignores that and keeps waiting until it times out). I’'ve filed a bug report with RhymBox regarding this problem and you may want to vote on that bug on their site to encourage them to fix it.