$umlauts = array (
  'ä',
  'Ä',
  'ü',
  'Ü',
  'ö',
  'Ö',
  'ß'
);
$umlauts_decoded = array (
  'ae',
  'Ae',
  'ue',
  'Ue',
  'oe',
  'Oe',
  'ss'
);

$_POST['Betreff']  = str_replace($umlauts, $umlauts_decoded, $_POST['Betreff']);
$_POST['Absender'] = str_replace($umlauts, $umlauts_decoded, $_POST['Absender']);