diff --git a/backend/setup.py b/backend/setup.py
index 9e4140b..443d3a3 100644
--- a/backend/setup.py
+++ b/backend/setup.py
@@ -30,6 +30,7 @@
         'pyramid_mailer',
         'pyramid_tm',
         'repoze.filesafe',
+        'repoze.sendmail < 4.2',
         'waitress',
         'zope.sqlalchemy',
     ],