EmailFinder must match on full name and address in order to return an email.  In addition, EmailFinder does not currently verify existing email addresses.  Therefore it is recommended that a query is used including only constituents with a street address, but not email address: 
Addresses, Preferred Address, Preferred Address Line 1 not blank
AND Phones, Phone Type not one of [select all phone types used for email addresses]

Note:It does not matter what fields are in Output in the query. The Data Health Center will choose specific fields while exporting based on the records pulled into the query.