Controllo su un campo completato

[Valeria75_bis]

Grazie per la risposta

Cosa sbaglio??

function controlla(){ 
   if( document.getElementById('mail') == ''){ 
      document.getElementById('submit2').disabled = true; 
   }    
      document.getElementById('submit2').disabled = false; 
   } 
}
</script>

<form action="InviaMail.php" method="post">
        <table width="274" border="0" align="center">
          <tr>
            <td><div align="right"><span class="Stile5">Nome*</span></div></td>
            <td><input id="nome" name="nome"></td>
          </tr>
          <tr>
            <td><div align="right"><span class="Stile5">Cognome*</span></div></td>
            <td><input id="cognome" name="cognome"></td>
          </tr>
          <tr>
            <td><div align="right" class="Stile5">Societ&agrave;/Qualifica</div></td>
            <td><input id="societa" name="societa"></td>
          </tr>
          <tr>
            <td><div align="right"><span class="Stile5">Indirizzo e-mail*</span></div></td>
            <td><input id="mail" name="mail" onchange="java-script:controlla()"></td>
          </tr>
          <tr>
            <td colspan="2"><div align="center">
                <input type="submit" name="submit2" value="Invia">
            </div></td>
          </tr>
        </table>
          </form>