When migrating away from Plesk I couldn't figure out an easy way to get a straight list of all e-mails, but here is where I found the solution:

http://nakuls77.wordpress.com/2008/08/30/details-of-email-ftp-database-users-from-psa-database/

mysql> SELECT mail.mail_name, accounts.password, domains.name FROM mail, accounts, domains WHERE domains.id=mail.dom_id AND mail.account_id=accounts.id;

mysql> SELECT CONCAT(mail.mail_name,’@’,domains.name) AS Email_Address , accounts.password AS Password FROM mail, domains, accounts WHERE domains.id=mail.dom_id AND mail.account_id=accounts.id;

This way you won't make the mistake of manually going through each domain and checking to see if there is an alias, forward or redirect buried somewhere inside Plesk.