This issue concerns Notifier on open-release/ginkgo.master. (Native install on Ubuntu Server 16.04).
I could not make Notifier work until I dig in the code and did a tiny change as follows
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In the env file
nano /edx/app/notifier/notifier_env
set the value
export EMAIL_BACKEND="django.core.mail.backends.smtp.EmailBackend"
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In settings file
nano /edx/app/notifier/src/notifier/settings.py
erased the whole EMAIL_BACKEND block, and changed the value to
EMAIL_BACKEND="django.core.mail.backends.smtp.EmailBackend"