Run this in the psa mysql database to see all email passwords
SELECT accounts.id, mail.mail_name, accounts.password, domains.name FROM domains LEFT JOIN mail ON domains.id = mail.dom_id LEFT JOIN accounts ON mail.account_id = accounts.id
Run this modified version to see just a particular domain
SELECT accounts.id, mail.mail_name, accounts.password, domains.name FROM domains LEFT JOIN mail ON domains.id = mail.dom_id LEFT JOIN accounts ON mail.account_id = accounts.id WHERE domains.name = 'somedomain.com'
Source: http://www.search-this.com/2007/02/07/view-all-email-account-passwords-in-plesk/