maybe you can post the code which causes this problem, I’'ve never seen such problems. And maybe some more lines of the stack trace. Please use and /code to mark your code as code.
There is also a KB article which describes why one may get a class cast exception: Why am I getting java.lang.ClassCastException?
So it should find all the stuff that your jar has.
authenticate calls bindResourceAndEstablishSession which breaks where I indicated (line 300).
I donno if it has anything to do with this, but I simply got a gmail mail account and using that to connect. (Donno if they require special things applied to account to allow logins to googletalk).