Email fail_silently, auth_user, auth_password are ignored when connection param provided

[Mike Edmunds]

Most django.core.mail APIs take a fail_silently parameter which is intended to suppress errors during sending, as well as a connection parameter to provide a specific EmailBackend instance for sending. When connection is specified, fail_silently is ignored.

To reproduce, configure your email settings to cause an error (e.g., EMAIL_HOST="not.a.real.host") and:

from django.core.mail import get_connection, send_mail

send_mail("subject", "body", None, ["to@example.com"])
# Correct behavior: raises socket.gaierror: [Errno 8] nodename nor servname provided, or not known

send_mail("subject", "body", None, ["to@example.com"], fail_silently=True)
# Correct behavior: returns 0 messages sent, doesn't raise an error

send_mail("subject", "body", None, ["to@example.com"], fail_silently=True, connection=get_connection())
# Bug: despite fail_silently=True, raises socket.gaierror

A similar problem exists with the auth_user and auth_password params to send_mail() and send_mass_mail(), which are also silently ignored when a connection is provided.

There is no safe way to set any of these options on a connection (EmailBackend instance) once it has been constructed. The correct way to achieve the desired behavior is move fail_silently and the other params into get_connection():

send_mail("subject", "body", None, ["to@example.com"],
    connection=get_connection(fail_silently=True, username="auth_user", password="auth_password"))

Suggested fix

Django's email APIs should handle fail_silently, auth_user and auth_password params as an error when connection is also provided. This will involve:

• Changing the default from fail_silently=False to fail_silently=None to be able to distinguish the cases. (Don't forget EmailMessage.send().)
• Raising a TypeError when connection is not None and fail_silently is not None. Something like "fail_silently cannot be used with a connection. (Pass fail_silently to get_connection() instead.)"
• Converting fail_silently==None to False where the high level APIs call get_connection()
• Issuing similar error messages for auth_user and auth_password (which already default to None)
• Updating tests to cover the new behavior