the name of you checkboxes is "Appointment_request_group_Confirm_by[]";
name="Appointment_request_group_Confirm_by[]"
your code is looking for an element named "Appointment_request_group_Contact_by":
$MailBody = $MailBody . ((isset($_POST["Appointment_request_group_Contact_by"]))?implode(", ", $_POST["Appointment_request_group_Contact_by"]):"");
change that to:
$MailBody = $MailBody . ((isset($_POST["Appointment_request_group_Confirm_by"]))?implode(", ", $_POST["Appointment_request_group_Confirm_by"]):"");