A kmail developer told me outright that kmail expects to be the only(!!!) application accessing the directory where the mail is stored in (wow that's bad). I fear all other modern/new/GUI clients are the same. A more satisfactory solution would be nice indeed. So far it's mutt.
... that's exactly my experience. All or most email client software has some sort of import filter to import MS Outlook, Mozilla, Netscape, Eudora or other well known mailboxes. But _none_ of them has an export function that produces some general format. :-(