Tästä
        $select = $this->select()
            ->from(array('bc' => 'building_contact') )
            ->joinLeft(array('b' => 'building'), 'b.id = bc.building_id', array('id'))
            ->joinLeft(array('c' => 'company'), 'c.id = b.company_id', array('email') )
            ->where('bc.email_sent IS NOT NULL')
            ->limit($limit);

pitäisi saada tämä:

SELECT
  bc.*,
  c.email
FROM
  building_contact bc LEFT JOIN building b ON b.id = bc.building_id
  LEFT JOIN company c ON c.id = b.company_id
WHERE
  AND bc.email_sent IS NULL;