German Umlaute mess up subject-line and body

These characters (ö, ä, ü and probably even others) in the subject-line lead to a multiplication of the [listname] and Re: when one replies to such a mail. This is not because of a crapy mailer. The more replies, the longer the subject-line gets (if the author doesn not delete it manually). The characters in the subject-line actually get displayed correctly. Unfortunately, the use of them in the body leads to an unreadable mess as has already been posted here earlier.
Needless to say that both these phenomenons are very annoying for actually at least every German list that runs mailman!