What does the Exchange Mailbox enumeration task do when it's scanning resource table?
During this period the gathering performs the main part of the enumeration of mailbox information. The Mailbox Resource Table is the collection of mailboxes that are returned from Exchange for a particular Exchange Server.
With this enumerated information, MessageStats takes that information and stores it into the database. For some of the information, like Mailbox Security, some additional calls are made to get the additional information.