We found out that it seems to be a problem with the libnet.
version 1.20 is installed on our web server, where we found the same problem, and on our Zimbra server.
From what we found libnet-1.20 converts 8-bit characters to UTF-8 without considering a declared MIME type (and possibly back).
We downgraded to 1.19 on our web server and now it works there, Will see if we can downgrade our Zimbra server.
UPDATE:
We do not need to downgrade our Zimbra server. It seems as the server is set to UTF-8 it works fine with libnet-1.20.
More can be found here: in
german or
english
Christian