Problema funcion mail

[karpoj23]

Para la validacion de emails tengo lo siguiente:

email = document.formulario.Email.value;
AtPos = email.indexOf("@");
StopPos = email.lastIndexOf(".");
longitud='';
vuelta='';
extension='';

longitud = email.length;

for (count=longitud; count >= 0; count--)

vuelta+=email.substring(count,count-1);

extension = vuelta.indexOf(".");

if ((longitud == (StopPos+1)) || (extension < 2 || extension >5))
{
Message = "The email address is not valid. Please check it.";
alert (Message);
return false;
}
if (AtPos == -1 || StopPos <= AtPos)
{
Message = "The email address is not valid. Please check it.";
alert (Message);
return false;
}

Es en Javascript.