Custom em_mailer_send_parameters filter

<?php

/*
 * Add admin emails (Events > Settings > Emails > Booking Email Templates > Email events admin)
 * as receiver when Resend button is clicked
 */
add_filter ('em_mailer_send_parameters', 'my_em_mailer_send_parameters', 10, 1 );
function my_em_mailer_send_parameters( $parameters ){
	if ( $_REQUEST['action'] == 'booking_resend_email' ){
	  $admin_emails = str_replace(' ','', get_option('dbem_bookings_notify_admin'));
	  $parameters['receiver'] = $parameters['receiver'] . ',' . $admin_emails;
	}
	return $parameters;
}