Step 1: Log in/access server via command line interface (try putty)

Step 2: Log into MYSQL
mysql psa -u admin -p`cat /etc/psa/.psa.shadow`

Step 3: Run this MYSQL query
select concat(mail.mail_name,'@',domains.name), password from mail left join accounts on account_id=accounts.id left join domains on domains.id=mail.dom_id where postbox='true' order by dom_id;