Hi
I think there is a bug when initial_response_ok=False and using AUTH PLAIN, the server will response like:
------------------
C: AUTH PLAIN
S: 334 ok. go on
------------------
and it's not base64 encoding, while in the auth() it will base64 decode the resp(here is "ok, go on") which will cause a binascii.Error:

Traceback (most recent call last):
  File "/usr/lib/python3.6/smtplib.py", line 644, in auth
    challenge = base64.decodebytes(resp)
  File "/usr/lib/python3.6/base64.py", line 553, in decodebytes
    return binascii.a2b_base64(s)
binascii.Error: Incorrect padding

I am using Magic Winmail Server2.4(build 0530) as a SMTP server, which appears dont support initial_response, so I set initial_response_ok=False and got this Error. 
currently I catch this error and ignore it to evade program failed, it works fine. Is there better way to fix this problem?

Thanks!

Technically, that is not the fault of smtplib.SMTP

The standard for SMTP AUTH specifies that characters following "334 " MUST be Base64 encoded.

See https://tools.ietf.org/html/rfc4954#page-4 , 3rd paragraph:

> A server challenge is sent as a 334 reply with the text part
> containing the [BASE64] encoded string supplied by the SASL
> mechanism.  This challenge MUST NOT contain any text other
> than the BASE64 encoded challenge.

Servers that send non-BASE64-encoded text after "334 " IMO is violating the standards.

A stronger case is the "Formal Syntax" on https://tools.ietf.org/html/rfc4954#page-13 :

>       continue-req    = "334" SP [base64] CRLF
>                         ;; Intermediate response to the AUTH
>                         ;; command.
>                         ;; This non-terminal complies with
>                         ;; syntax defined by Reply-line [SMTP].

Nothing else besides base64 is allowed; after "334 " it MUST be either <base64> or <nothing>.

> I am using Magic Winmail Server2.4(build 0530) as a SMTP server, which appears dont support initial_response, so I set initial_response_ok=False and got this Error. currently I catch this error and ignore it to evade program failed, it works fine. Is there better way to fix this problem?

I suggest either of:

(1) Contacting the makers of the software, to force Base64-encoding for "334 " replies, or

(2) If the text following "334 " is customizable, Base64-encode them yourself, then use the base64-encoded text as the customized "334 " response.

Hi Pandu, thanks for your reply!
I have read the RFC4954 you mentioned and agree that the server shouldn't react like this. Then I notice that this RFC's publication date is 2007 and the server I use is published in 2003, that's maybe the reson why I meet this problem, Maybe I should just evade this problem for my own program and close this bug?