Update (almost) all the docs

docs/tips/multiple_backends.rst

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+.. _multiple-backends:
+
+Mixing email backends
+=====================
+
+Since you are replacing Django's global :setting:`EMAIL_BACKEND`, by default
+Anymail will handle **all** outgoing mail, sending everything through your ESP.
+
+You can use Django mail's optional :func:`connection <django.core.mail.get_connection>`
+argument to send some mail through your ESP and others through a different system.
+
+This could be useful, for example, to deliver customer emails with the ESP,
+but send admin emails directly through an SMTP server:
+
+.. code-block:: python
+    :emphasize-lines: 8,10,13,15
+
+    from django.core.mail import send_mail, get_connection
+
+    # send_mail connection defaults to the settings EMAIL_BACKEND, which
+    # we've set to Anymail's MailgunBackend. This will be sent using Mailgun:
+    send_mail("Thanks", "We sent your order", "sales@example.com", ["customer@example.com"])
+
+    # Get a connection to an SMTP backend, and send using that instead:
+    smtp_backend = get_connection('django.core.mail.backends.smtp.EmailBackend')
+    send_mail("Uh-Oh", "Need your attention", "admin@example.com", ["alert@example.com"],
+              connection=smtp_backend)
+
+    # You can even use multiple Anymail backends in the same app:
+    sendgrid_backend = get_connection('anymail.backends.sendgrid.SendGridBackend')
+    send_mail("Password reset", "Here you go", "user@example.com", ["noreply@example.com"],
+              connection=sendgrid_backend)
+
+You can supply a different connection to Django's
+:func:`~django.core.mail.send_mail` and :func:`~django.core.mail.send_mass_mail` helpers,
+and in the constructor for an
+:class:`~django.core.mail.EmailMessage` or :class:`~django.core.mail.EmailMultiAlternatives`.
+
+
+(See the :class:`django.utils.log.AdminEmailHandler` docs for more information
+on Django's admin error logging.)
+
+.. _django.utils.log.AdminEmailHandler:
+    https://docs.djangoproject.com/en/stable/topics/logging/#django.utils.log.AdminEmailHandler