diff --git a/addons/mass_mailing/models/mailing.py b/addons/mass_mailing/models/mailing.py
index a756620a4f0..968c0c373b6 100644
--- a/addons/mass_mailing/models/mailing.py
+++ b/addons/mass_mailing/models/mailing.py
@@ -536,7 +536,7 @@ class MassMailing(models.Model):
             ('model', '=', self.mailing_model_real),
             ('res_id', 'in', res_ids),
             ('mass_mailing_id', '=', self.id)], ['res_id'])
-        done_res_ids = [record['res_id'] for record in already_mailed]
+        done_res_ids = {record['res_id'] for record in already_mailed}
         return [rid for rid in res_ids if rid not in done_res_ids]
 
     def _get_unsubscribe_url(self, email_to, res_id):