Document server error when missing Mailgun config
Fixes
@@ -5,6 +5,8 @@ This page contains some advice about errors and problems commonly encountered du
#. ``project_slug`` must be a valid Python module name or you will have issues on imports.
-#. ``jinja2.exceptions.TemplateSyntaxError: Encountered unknown tag 'now'.``: please upgrade your cookiecutter version to >= 1.4 (see # 528_)
+#. ``jinja2.exceptions.TemplateSyntaxError: Encountered unknown tag 'now'.``: please upgrade your cookiecutter version to >= 1.4 (see `#528`_)
-.. _528: https://github.com/pydanny/cookiecutter-django/issues/528#issuecomment-212650373
+#. Internal server error on user registration: make sure you have configured the mail backend (e.g. Mailgun) by adding the API key and sender domain
+
+.. _#528: https://github.com/pydanny/cookiecutter-django/issues/528#issuecomment-212650373
