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Also, If there is doc on querying via webservices (REST or SOAP) that would possibly work, but some SQL based query is preferred.
Background: We use a middleware product called Handshake that allows us to profile a database and query it and display that data on our SharePoint server, etc.
The short answer from one of the internal Zimbra DB experts is that it is not viable, since to, from, cc, bcc, and other metadata is (with one or two exceptions) not saved in the database. Bummer.