Just stumbled upon this bug-report, that exactly described a problem I had for some time with kmail + imap on my “web.de”-account
http://bugs.kde.org/show_bug.cgi?id=153284
The proposed solution worked fine for me, so no more hanging inbox, fyi what to edit and where:
Despite being unintuitive, you have to use the edit-icon, changing the input field directly will result in [...]