if( mail('email@website.de', 
    mb_encode_mimeheader($_POST['Betreff'], "UTF-8", "Q"), 
    $_POST['Text'], 
    'From: ' . mb_encode_mimeheader($_POST['Absender'], "UTF-8", "Q") . "\n" 
      . 'Content-Type: text/plain; charset="utf-8"' . "\n" 
      . 'Content-Transfer-Encoding: 8bit' )
    ) {