Thank you, Jason!
I know using utf8 could be a solution for this part of the problem ...
But the reason why I use sometimes "ISO-8859-1" is that some older email clients don't correctly display the "special german characters" in contact-form-messages if the charset of the site is "utf8" (I guess this are older versions of Outlook or so, which can't handle utf8 ?!?).
However, since I don't know an easy solution I could give to my clients to change that I do what I can do on my side and use the charset "ISO-8859-1" instead. Then these messages display correctly.
Thatswhy my question: is there a way to resolve that issue WITH using "ISO-8859-1" ?