[2019-06-16 19:35:00,092: ERROR/MainProcess] Received unregistered task of type 'apps.mailing.tasks.execute_sending_system_mail'.
The message has been ignored and discarded.

Did you remember to import the module containing this task?
Or maybe you're using relative imports?

Please see
http://docs.celeryq.org/en/latest/internals/protocol.html
for more information.

The full contents of the message body was:
'[[], {}, {"callbacks": null, "errbacks": null, "chain": null, "chord": null}]' (77b)
Traceback (most recent call last):
  File "/home/michael93pl/apps/venv/loans/lib/python3.7/site-packages/celery/worker/consumer/consumer.py", line 557, in on_task_received
    strategy = strategies[type_]
KeyError: 'apps.mailing.tasks.execute_sending_system_mail'