if( mail('email@website.de', 
    utf8_decode($_POST['Betreff']), 
    $_POST['Text'], 
    'From: ' . utf8_decode($_POST['Absender']) . "\n" 
      . 'Content-Type: text/plain; charset="utf-8"' . "\n" 
      . 'Content-Transfer-Encoding: 8bit' )
    ) {