What is the email of the customers who place their orders with priority method?
priority method refers to method_name = 'Priority'
SELECT T1.email FROM customer AS T1 INNER JOIN cust_order AS T2 ON T1.customer_id = T2.customer_id INNER JOIN shipping_method AS T3 ON T3.method_id = T2.shipping_method_id WHERE T3.method_name = 'Priority'