TLS/SSL sending mail fails in some servers

[raydeal]

After last changes in django.core.mail.backends.smtp.EmailBackend sending email is broken in some mail servers. There is an error ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:992)

According to Python documentation ​SSL security and ​create_default_context I think that working version of implementation is

@cached_property
def ssl_context(self):
    if self.ssl_certfile:
       ssl_context = ssl.SSLContext(protocol=ssl.PROTOCOL_TLS_CLIENT)
       ssl_context.load_cert_chain(self.ssl_certfile, self.ssl_keyfile)
    else:
       ssl_context = ssl.create_default_context()
    return ssl_context

Additionally it would be great to clarify in documentation of Django settings what is EMAIL_SSL_CERTFILE and EMAIL_SSL_KEYFILE because a key and certificate can be in one file passed as EMAIL_SSL_CERTFILE and it is enough to pass certificate validation.

[Claude Paroz]

Did you test before or after this change: [b9d89d74f71c043d5269c]?

[Tim Graham]

The proposed implementation in the description matches the code in the commit that Claude mentioned, so marking as a duplicate of #34386.

raydeal, feel free to propose a documentation patch regarding your point in the last sentence. Such a small change doesn't require a ticket.