Based on Django documentation:

During test running, each outgoing email is saved in django.core.mail.outbox. This is a list of all EmailMessage instances that have been sent. The outbox attribute is a special attribute that is created only when the locmem email backend is used. It doesn’t normally exist as part of the django.core.mail module and you can’t import it directly.

Could you please add a test to show in what condition the above mentioned error happens?

The above error is from running test suite of https://github.com/evansd/whitenoise

Complete build log contains records of all packages used and steps taken.

@evansd Does this whole ticket make sense to you?

This sounds like an incompatibility between the versions of Django and pytest-django being used and nothing specific to whitenoise.

Im having the Same errors, i created a custom django_db_setup() fixture and give my database definition to the settings.configure(DATABASES=)
Im using pytest 7.0.1, pytest-django 4.2.5
I dont even use The Django email stuff, its just a basic graphene application.

I see the same issue occurring recently. Note that the error occurs in the _dj_autoclear_mailbox test fixture that is marked with autouse=True. So it occurs with any test.

I my case, it occurs in a project in which Django is set up manually because the project only needs to be able to generated html based on Django templates. For testing, the Django instance is set up as follows:

@fixture(autouse=True, scope="session")
def django():
    import django
    from django.conf import settings

    settings.configure(
        TIME_ZONE="Europe/Brussels",
        USE_I18N=True,
        USE_L10N=False,
    )
    django.setup()

Note that I can avoid the issue by adding the following in my root conftest:

@fixture(scope="function", autouse=True)
def _dj_autoclear_mailbox() -> None:
    # Override the `_dj_autoclear_mailbox` test fixture in `pytest_django`.
    pass

Reproduced with Django 4.2.7 with no settings configuration. For me, I happened to have Django in my venv but had not configured a settings in this particular test slice and ran into the exception.

Thanks @wouter-vd, your workaround did indeed fix it!