Simple and to the point; query the db for domains with email enabled. This is only tested on Plesk 11.5:

SELECT domains.name, DomainServices.*
FROM domains, DomainServices 
WHERE DomainServices.type = 'mail' 
 AND DomainServices.dom_id = domains.id 
 AND DomainServices.status = '0' 
ORDER BY name ASC;