public function send_tickets_email( $order_id, $event_id = null ) {
		$all_attendees = $this->get_attendees_by_order_id( $order_id );

		$to_send = array();

		if ( empty( $all_attendees ) ) {
			return;
		}

		// Look at each attendee and check if a ticket was sent: in each case where a ticket
		// has not yet been sent we should a) send the ticket out by email and b) record the
		// fact it was sent
		foreach ( $all_attendees as $single_attendee ) {
			// Do not add those attendees/tickets marked as not attending (note that despite the name
			// 'qr_ticket_id', this key is not QR code specific, it's simply the attendee post ID)
			$going_status = get_post_meta( $single_attendee['qr_ticket_id'], self::ATTENDEE_RSVP_KEY, true );
			if ( in_array( $going_status, $this->get_statuses_by_action( 'count_not_going' ), true ) ) {
				continue;
			}

			// Only add those attendees/tickets that haven't already been sent
			if ( empty( $single_attendee['ticket_sent'] ) ) {
				$to_send[] = $single_attendee;
				update_post_meta( $single_attendee['qr_ticket_id'], self::ATTENDEE_TICKET_SENT, true );
			}
		}

		/**
		 * Controls the list of tickets which will be emailed out.
		 *
		 * @param array $to_send        list of tickets to be sent out by email
		 * @param array $all_attendees  list of all attendees/tickets, including those already sent out
		 * @param int   $order_id
		 *
		 */
		$to_send = (array) apply_filters( 'tribe_tickets_rsvp_tickets_to_send', $to_send, $all_attendees, $order_id );

		if ( empty( $to_send ) ) {
			return;
		}

		// For now all ticket holders in an order share the same email
		$to = $all_attendees['0']['holder_email'];

		if ( ! is_email( $to ) ) {
			return;
		}

		/**
		 * Filters the RSVP tickets email headers
		 *
		 * @since 4.5.2 added new parameters $event_id and $order_id
		 *
		 * @param array  email headers
		 * @param int    $event_id
		 * @param int    $order_id
		 */
		$headers = apply_filters( 'tribe_rsvp_email_headers', array( 'Content-type: text/html' ), $event_id, $order_id );

		/**
		 * Filters the RSVP tickets email attachments
		 *
		 * @since 4.5.2 added new parameters $event_id and $order_id
		 *
		 * @param array  attachments
		 * @param int    $event_id
		 * @param int    $order_id
		 */
		$attachments = apply_filters( 'tribe_rsvp_email_attachments', array(), $event_id, $order_id );

		/**
		 * Filters the RSVP tickets email recipient
		 *
		 * @since 4.5.2 added new parameters $event_id and $order_id
		 *
		 * @param string  $to
		 * @param int     $event_id
		 * @param int     $order_id
		 */
		$to = apply_filters( 'tribe_rsvp_email_recipient', $to, $event_id, $order_id );

		/**
		 * Filters the RSVP tickets email subject
		 *
		 * @since 4.5.2 added new parameters $event_id and $order_id
		 *
		 * @param string
		 * @param int     $event_id
		 * @param int     $order_id
		 */
		$subject     = apply_filters( 'tribe_rsvp_email_subject',
			sprintf( __( 'Your tickets from %s', 'event-tickets' ), stripslashes_deep( html_entity_decode( get_bloginfo( 'name' ), ENT_QUOTES ) ) ),
			$event_id,
			$order_id
		);

		/**
		 * Filters the RSVP tickets email content
		 *
		 * @since 4.5.2 added new parameters $event_id and $order_id
		 *
		 * @param string  email content
		 * @param int     $event_id
		 * @param int     $order_id
		 */
		$content = apply_filters( 'tribe_rsvp_email_content', $this->generate_tickets_email_content( $to_send ), $event_id, $order_id );

		wp_mail( $to, $subject, $content, $headers, $attachments );
	}