do not include the square brackets. change:
$MailBody = $MailBody . ((isset($_POST["Appointment_request_group_Type_of_event[]"]))?implode(", ", $_POST["Appointment_request_group_Type_of_event[]"]):"");
to:
$MailBody = $MailBody . ((isset($_POST["Appointment_request_group_Type_of_event"]))?implode(", ", $_POST["Appointment_request_group_Type_of_event"]):"");