smtp EmailBackend doesn't honor fail_silently=True when receiving a socket level connection error

[Tom Hendrikx]

The Python SMTPlib does not wrap connection errors that occur on socket level while setting up a connection. That means that Django needs to catch socket errors too (@here: ​https://github.com/django/django/blob/master/django/core/mail/backends/smtp.py#L73) when fail_silently is enabled and the connection fails with a socket.error (f.i.: Connection refused) or socket.timeout (f.i. Connection timed out).

I tested this by setting up a django 1.10 with following settings:

EMAIL_BACKEND = 'django.core.mail.backends.smtp.EmailBackend'
EMAIL_HOST = 'localhost'
EMAIL_PORT = 10587  # nothing is listening here!

Then I copied the core admincommand sendtestemail.py to one of my apps, and modified ​https://github.com/django/django/blob/master/django/core/management/commands/sendtestemail.py#L34 to set fail_silently=True. This should yield no error, but it ends in a Traceback.

Original problem of this issue for me was the utils.log.AdminEmailHandler that uses this feature. I can try to create some test cases for this issue when the issue is deemed valid.

[Vesteinn Snaebjarnarson]

See pull request at ​https://github.com/django/django/pull/7282

Added ConnectionError to the exception tuple for the SMTPBackend with tests.

[Tim Graham]

Please uncheck "Patch needs improvement" when review comments are addressed and tests are passing.

[Tim Graham <timograham@…>]

In 602bffe7:

Fixed #27210 -- Allowed SMTPBackend to fail silently on a socket connection error.

[GitHub <noreply@…>]

In f6fe8ecc:

Refs #27210 -- Fixed isolation of test_fail_silently_on_connection_error.

The test wouldn't pass if a mail server is running on the system.