Well I splitted my lines of the header with \r\n instead of only \n but I'm note sure if there was a change or not...
So my header var looks e.g. sth like this, if I don't use Pear's Mail class.
$header = "MIME-Version: 1.0\r\n";
$header .= "Content-type: text/html; charset=iso-8859-1\r\n";
$header .= "From: email@example.com <firstname.lastname@example.org>\r\n";