una cosa del genere:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<title>Pagina senza titolo</title>
<script type="text/javascript">
function _Destinatari(_Mail){
var _destinatario = document.getElementById("Destinatario").value
if(_Mail == "Resetta"){
document.getElementById("Destinatario").value='';
document.getElementById("Email").selectedIndex=0;
}else if(_Mail == 0){
alert('Valore non valido riprovare');
return false;}
else if(_Mail != 0 && _destinatario == ""){
document.getElementById("Destinatario").value = _Mail;
}else{
document.getElementById("Destinatario").value+=";\n"+_Mail}}
</script>
</head>
<body>
<form action="form1" method="post"><table width="100" border="0" cellspacing="0" cellpadding="2">
<tr>
<td>Email</td>
<td><select name="Email" id="Email" onchange="_Destinatari(this.options[this.selectedIndex].value)">
<option value="0">Seleziona destinatari</option>
<option value="RossiMario@libero.it">Rossi Mario</option>
<option value="BianchiLuigi@tin.it">Bianchi Luigi</option>
<option value="VerdiStefano@tiscali.it">Verdi Stefano</option>
<option value="NeriGiovanni@email.it">Neri Giovanni</option>
</select></td>
</tr>
<tr>
<td>Destinatario</td>
<td><textarea name="Destinatario" cols="50" rows="5" readonly="readonly" id="Destinatario"></textarea></td><td>Reset</td>
</tr>
</table>
</form>
</body>
</html>