I don’t know about the K790i but I found that the included client with the E61 did not work with Jabber. I have tried eqo, fring (both have extra voice capabilities), Talkonaut, Agile Messenger, IM+ and one other that I cannot remember! I decided in the end that I preferred IM+. It works on wide range of phones and certainly works on the E61. However, I think the K790i would require the java version. It is not free but it is not particularly expensive. You can try all of them out for yourself. All of the ones I have mentioned also connect with other IM services.
Agile messenger no longer seems to offer jabber but if you set up another service, eg gtalk then xmpp miraculously appears!