class SMTP:
    def auth_login(self, challenge=None):

The self.docmd should use cmd "AUTH" with parameter "LOGIN" + encoded login like

        (code, resp) = self.docmd("AUTH", "LOGIN " +
            encode_base64(self.user.encode('ascii'), eol=''))

with

    def auth(self, mechanism, authobject, *, initial_response_ok=True):

that should not send a "AUTH" in self.docmd in case the mechanism is 'LOGIN' and

        if initial_response is not None:

meaning

            if mechanism == 'LOGIN':
                (code, resp) = self.docmd(response)
            else:
                (code, resp) = self.docmd("AUTH", mechanism + " " + response)

---

Could someone kindly review, evtly come up with better suggestion?

In short:
$ diff /c/Python35/Lib/smtplib-old.py /c/Python35/Lib/smtplib.py
630c630,633
<             (code, resp) = self.docmd("AUTH", mechanism + " " + response)
---
>             if mechanism == 'LOGIN':
>                 (code, resp) = self.docmd(response)
>             else:
>                 (code, resp) = self.docmd("AUTH", mechanism + " " + response)
660c663
<         (code, resp) = self.docmd(
---
>         (code, resp) = self.docmd("AUTH", "LOGIN " +