If you forgot your Plesk admin password, connect to your server with ssh and check /etc/psa/.psa.shadow file.
[root@apollo ~]# cat /etc/psa/.psa.shadow
<adminpassword>
To find mailbox’s password in Plesk, connect to your server with ssh, then issue the following SQL query to reveal mailbox password (replace MAILBOX with your mailbox name):
[root@apollo ~]# mysql -u admin -p`cat /etc/psa/.psa.shadow` psa
[...]
mysql> SELECT m.mail_name, d.name, a.password FROM mail AS m \
LEFT JOIN (domains AS d, accounts AS a) \
ON (m.dom_id = d.id AND m.account_id = a.id) WHERE m.mail_name='MAILBOX';