diff --git a/welco/sources/mail/__init__.py b/welco/sources/mail/__init__.py
index 60d86e0..0304d62 100644
--- a/welco/sources/mail/__init__.py
+++ b/welco/sources/mail/__init__.py
@@ -37,7 +37,7 @@ class AppConfig(django.apps.AppConfig):
         if not instance.formdata_id:
             return
         source = instance.source
-        if not hasattr(source, 'external_id'):
+        if not getattr(source, 'external_id', None):
             return
         external_id = source.external_id
         if not external_id.startswith('maarch-'):