Since a few months I have problems with gmx.net's XMPP service (note that this is the same company that provides gmx.com, but it's not the same service).
It doesn't update my roster anymore, so if a contact goes on/off in the meantime, I can't see it until I reconnect. Also I get sometimes "Remote server not found" errors when trying to send messages to contacts on jabber.org (offline message seem not to work anymore at all).
I searched for this issue and found a mailing list thread, where Kevin Smith says:
I believe this is another example (there have been many) of an ejabberd server running on an old OS. Ejabberd servers on an old OS seem to not be able to cope with the new certificate installed on jabber.org. Upgrading the OS seems to fix this (without upgrading ejabberd) - the current suspect is an old version of openssl.
Does anyone having no problems with XMPP at gmx.net? Or is this really a global problem on GMX's side? As I have this problem for several months now (worked perfectly fine for several years before), it's hard to believe that GMX doesn't know about and fix it. On the other hand, XMPP is probably not that widely used by their users (I guess most use it for email only).
I tried contacting the support some time ago (other issue related to XMPP), but never got a response. Now I can't find a contact form or email address, only an expensive phone number :/