Version: 3.5.1 (using KDE KDE 3.5.1) Installed from: Gentoo Packages OS: Linux If I receive a Outlook calendar message with encoding iso-8859-1, kmail correctly displays the message. But on sending the response, the encoding is always changed to utf-8, no matter what settings I have configured. You can test this by sending an appointment from Outlook with umlauts (äöü...) in the subject and then accept/decline it in KMail. Outlook will not get the umlauts in the response.
Can you check if the Subject line in the mail source (pressing "V" or View Source) starts with the following: Subject: =?iso-8859-1? (The Subject line should have this special notation when it contains Umlauts but some (older) mail clients do not produce this correctly. Maybe this is the problem)
I fixed this today in KDE 3.5 and KDE4 plugins/kmail/bodypartformatter/text_calendar.cpp