[Exim] why is exim adding headers?

Hello,

I was almost running crazy with the Perl-Module MIME::Lite while I
thought that it was adding headers. But then I found out, that it wasn't
the module, it was exim! Although I'm calling exim (through a symlink at
/usr/sbin/sendmail) with the option -flehmann@???, two headers are
added, that I don't want to see:

Return-Path: nobody@???
Sender: Nobody <nobody@???>

It seems to be because of the user "nobody" at which the webserver and
the perl-script is running. These headers aren't added when the emails
are sent through any other user. Why is that? How can I disable it? It
looks ugly and is completely useless (at least for me).

Regards
Marten