Well, like I said, shot in the dark. If you haven't touched them then it is indeed doubtful this is your problem.
To be sure, a simple:
$ chown -R <your username> ~/.kde
...will make sure all the files are owned by you.
Maybe time to search bugs.kde.org? Also, it is not out of the realm of possibility that this is a problem on the server side.
If I was to troubleshoot this further, I would try using a different mailer (ie: mutt, thunderbird etc) and see if they can connect fine. Narrow down the problem as much as possible.