What is the customer id for that mailshot id?
SELECT T1.customer_id FROM mailshot_customers AS T1 JOIN customers AS T2 ON T1.customer_id   =   T2.customer_id GROUP BY T1.customer_id ORDER BY count ( * )  DESC LIMIT 1
customer id for that mailshot id refers to selecting customer_id from a table that combines mailshot_customers and customers based on matching customer_id; the most frequently occurring customer_id in relation to the mailshots.