Fix invalid function call causing fatal error.

civicrm_mailer.http.class.php

@@ -62,7 +62,7 @@ class CivicrmMailerHTTPMailer extends \PHPMailer
       // The user might not exist.
       // Find a matching email in CiviCRM, prioritize primary emails.
       if (empty($contact)) {
-        $dao = CRM_Core_DAO::executeQuery('SELECT contact_id FROM civicrm_email WHERE email = %1 AND contact_id IS NOT NULL ORDER BY is_primary DESC', [
+        $dao = \CRM_Core_DAO::executeQuery('SELECT contact_id FROM civicrm_email WHERE email = %1 AND contact_id IS NOT NULL ORDER BY is_primary DESC', [
           1 => [$recipient, 'String'],
         ]);