beh.. non so bene cosa intendi con firma, però se ho inteso bene.. potresti fare così

Codice PHP:
$firma = "--------------------\nPico della Mirandola - Umanis\n";
$messaggio = "<html>
<head>
  <title>Birthday Reminders for August</title>
</head>
<body>
  

Here are the birthdays upcoming in August!</p>
  <table>
    <tr>
      <th>Person</th><th>Day</th><th>Month</th><th>Year</th>
    </tr>
    <tr>
      <td>Joe</td><td>3rd</td><td>August</td><td>1970</td>
    </tr>
    <tr>
      <td>Sally</td><td>17th</td><td>August</td><td>1973</td>
    </tr>
  </table>
  $firma
</body>
</html>";