_sent_unpaid_order_notification

send_unpaid_order_notification

$meta_query = array(
	'relation' => 'OR',
	array(
		'key'     => '_send_unpaid_order_notification',
		'value'   => 'yes',
		'compare' => '!=',
	),
	array(
		'key'     => '_send_unpaid_order_notification',
		'compare' => 'NOT EXISTS',
	),
);