Larry and others, I'd like to bring your attention to issue #15014. This issue added arbitrary auth methods to smtplib, which is a good thing. Implicitly though, a regression was introduced w.r.t. RFC 4954's optional initial-response for the AUTH command, for authentication methods that support it.
An example is AUTH PLAIN, which does not require a challenge. It's possible that SMTP servers are not prepared to handle challenge/responses for authentication methods that support initial-response, and regressions have been seen in the wild while testing against Python 3.5. In Python 3.4, AUTH PLAIN included an initial-response. After discussion on the issue, RDM and I agreed on a refinement to the authobject() protocol, such that they would be called first with challenge=None. If the auth method implemented by the authobject() supports an initial response, it can return the bytes to be encoded and sent along with AUTH <METHOD>. If it returns None, then a challenge is required, and the normal SMTP challenge/response can proceed. A minor complication is that clients using smtplib might want to force challenge/response instead of initial-response even for authentication methods that support the latter. There are various reasons for this, including test suites (such as Python's own test_smtplib.py). So I added an optional keyword argument called *initial_response_ok* to SMTP.auth() and SMTP.login(), with a default value of True. Thus for authentication methods that support it, smtplib will by default send an initial-response, but it can easily be overridden. Defaulting to True restores compatibility with Python 3.4. Technically, this is a new feature even though it addresses a regression in Python 3.5. Assuming a positive response by RDM or anybody else who would like to review the patch, I'd like to get a beta release exemption for the change. Patch on the issue includes implementation, test, and docs. Cheers, -Barry
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