<script language="javascript">
function validarMail(id){
var mail = eval('document.FrmMails.mail'+id+'.value');
var reg = new RegExp("^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$", "i");
if(mail.search(reg))
alert('La dirección es válida');
else
alert('La dirección es inválida');
}
</script>
<body>
<form name="FrmMails">
<input type="text" name="mail0" value=""> <input type="checkbox" onclick="javascript:validarMail(0)"><br>
<input type="text" name="mail1" value=""> <input type="checkbox" onclick="javascript:validarMail(1)"><br>
<input type="text" name="mail2" value=""> <input type="checkbox" onclick="javascript:validarMail(2)"><br>
</form>
</body>