To find mailboxes by client in Plesk

mailboxes_by_client.sql

SELECT mail.mail_name,domains.name
FROM  clients, domains, mail
WHERE clients.id = <integer>
AND clients.id = domains.cl_id
AND domains.id = mail.dom_id
ORDER BY domains.name;

mailboxes_by_client_w_passwords.sql

SELECT mail.mail_name,domains.name, SUBSTRING(a.password, 1) AS pw
FROM  clients, domains, mail, accounts a
WHERE clients.id = <integer>
AND clients.id = domains.cl_id
AND domains.id = mail.dom_id
AND a.id = mail.account_id
ORDER BY domains.name;