Yes, but here, I define the "from-mail" in the settings.

OK, I see! What about setting the forum e-mail as From: and the users's e-mail as Reply-to:?

To do so, replace ...

else
 {
  $headers  = "From: " . $from . MAIL_HEADER_SEPARATOR;
 }

... by ...

else
 {
  $headers = "From: " . $settings['forum_email'] . MAIL_HEADER_SEPARATOR;
  $headers .= "Reply-to: " . $from . MAIL_HEADER_SEPARATOR;
 }

in includes/functions.inc.php, about line 1981 (untested!).

Alex