I have been using kmail with the maildir format for some time now and have had no problems. I have quite a few folders other than the standard inbox etc. One such folder is called eg. oldbackup, which was created by kmail.
Now, however, I would like to access this directory from mutt and find that kmail uses a different directory structure than expected. The directory ~/Mail/oldbackup exists and has the directories cur, new and tmp, however these directories are empty. Instead there is another directory called .oldbackup.directory, which contains my mail. This directory cannot be easily found by mutt. Is it that I have misunderstood the maildir format, or is it that kmail uses a proprietory format? If so, is this documented somewhere? The faq section 6.6 implies to me that kmail using maildir should be compatible with mutt.
Another test I tried, was copy a maildir, which was written to by procmail (which supports maildir), to my ~/Mail directory. Kmail seems to support this fully. So I think the problem is with folders that are created by kmail. It should not be too difficult to manually rename the directories so that both mail clients can read the same mail, but I would like to know if others see this as a bug or a feature.