function verifyString (str, fieldName)
{
	if (str == null || str == "")
	{
		alert ("You have to fill in the " + fieldName);
		return false;
	}
	return true;
}

function verifyEmailAddr (str, fieldName)
{
	 if (!verifyString (str, fieldName))
	   return false;
	var i = str.indexOf ("@");
	if (i == -1)
	{
		alert ("Enter valid " + fieldName);
		return false;
	}
	return true;
}

function validateMessage (form)
{
	return verifyEmailAddr (form.Sender.value, "Sender e-mail address") &&
		   verifyEmailAddr (form.Recipient.value, "Recipient e-mail address") &&
		   verifyString (form.Subject.value, "Message Subject") &&
		   verifyString (form.Body.value, "Message body");
}