Kai, I hate to bother you, but there's one problem with your Gnus
setup: it lies about the character set your messages are using.  The
headers say:

  Content-Type: text/plain; charset=us-ascii

However, your name, Kai Großjohann, in the From: header uses Latin-1
characters.  This might go unnoticed by people whose Mule setup is the
default one, where Latin-1 is the default non-ASCII character set, but
my setup uses ISO 8859-8 set as the default, and therefore I see funny
characters in your name.

Could you please tell Gnus to say "charset=iso-8859-1" in the headers?