use change when you change the selected item and
$(document).ready(function () {
change($("#ddlMail"));
$("#ddlMail").change(function () {
change($(this));
});
});
function change(obj){
if ($(obj).val() == "Y") {
$('#lblC').show();
$('#ddlC').show();
}
else{
$('#lblC').hide();
$('#ddlC').hide();
}
}