Grazie ma io non volevo che si aprisse outlook - nuovo messaggio, io ho scritto così:
<html>
<head>
<SCRIPT language="Javascript">

function destin() {

var destinatario = document.modulo.destinatario.options[document.modulo.destinatario.selectedIndex].value;

funz_destinatario = "mailto:" + destinatario + "?subject=prova"; // soggetto dell'email

document.forms[form].action = funz_destinatario;
}
</SCRIPT>
</head>
<body>

<FORM ENCTYPE="text/plain" NAME="form" METHOD='post' ACTION='' onSubmit="return destin()">

Write to: <select name="destinatario" >
<option value="pippo@gmail.com">pippo</option>
<option value="pluto@gmail.com">pluto</option>
<option value="paperino@gmail.com">paperino</option>
</select>

<label>Write your message

<textarea name="messaggio" cols="30" rows="10"></textarea>


</label>
<input type="submit" value="Spedisci">
<input type="reset" value="Reset">
</form>
</body>
</html>