bernd,
sorry if I can across obnoxiously. It is one of my not so nice traits.
If you are on the latest packages, make a backup of your postfix directory, fill out the blanks on the mail configuration page, and see, if stuff ends up in the postfix directory. If it does, diff it to the stuff you created manually, so we can see the difference. If no files end up in the postfix configuration directory, I'd like to get the output of
dpkg -l pluto-website-admin
Thanks.