I have a patch to support initial-response, which I'll be posting here after a bit of clean up and a full (local) test run, with documentation.  I ended up adding a keyword argument `initial_response_ok=True` to .login() and .auth().  The reason for this is that some clients may wish to force smtplib to do challenge/response.

By default, mechanisms that support it (e.g. AUTH PLAIN) will send the initial response.  Setting `initial_response_ok=False` in either SMTP.auth() or SMTP.login() will prevent smtplib from sending the initial response, and instead deal with challenge/response.

I made initial_response_ok a keyword argument.