bug#48562: 27.1; smtpmail does not attempt alternative auth methods when primary (cram-md5) fails

[klu]

For some reason the SMTP server I talk to (mx.sdf.org) fails the CRAM-MD5
authetication, and the bug is, when this happens, smtpmail didn't know to try
the rest of the supported auth methods (namely, 'plain, and 'login), and
therefore, fails to send the mail.

The SMTP info is:

  (setq smtpmail-smtp-server "mx.sdf.org")
  (setq smtpmail-smtp-service 587)
  (setq mtpmail-stream-type 'starttls)


And here is smtpmail's trace on the failure - notice how it stopped after
CRAM-MD5 failure and didn't try the rest:

  220 mx.sdf.org ESMTP Sendmail 8.15.2/8.14.5; Thu, 20 May 2021 23:55:20 GMT
  250-mx.sdf.org Hello xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [xx.xxx.xxx.xx],
                 pleased to meet you
  250-ENHANCEDSTATUSCODES
  250-PIPELINING
  250-EXPN
  250-VERB
  250-8BITMIME
  250-SIZE 2147483648
  250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN
  250-STARTTLS
  250-DELIVERBY
  250 HELP
  220 2.0.0 Ready to start TLS
  250-mx.sdf.org Hello xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 
                 pleased to meet you
  250-ENHANCEDSTATUSCODES
  250-PIPELINING
  250-EXPN
  250-VERB
  250-8BITMIME
  250-SIZE 2147483648
  250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN
  250-DELIVERBY
  250 HELP
  AUTH CRAM-MD5
  334 PDM1O___________________________________Zz4=
  a2___________________________________________TAx
  535 5.7.0 authentication failed
  QUIT
  221 2.0.0 mx.sdf.org closing connection
  Process smtpmail connection broken by remote peer


This is clearly Emacs problem. Because I can send mail using other clients, 
including
iOS mail app, mac mail app, Alpine, and, below as an example, hand-made Python 
code:


The code:

  >>> import smtplib as s
  >>> with s.SMTP("mx.sdf.org", 587) as smtp:
  ...     smtp.set_debuglevel(2)
  ...     smtp.login('foobar', '___________')
  ...     smtp.sendmail(
  ...         "From: foobar@sdf.org",
  ...         "To: foobar@example.com",
  ...         "hello"
  ...     )
  ... 


The log (notice how Python also fails CRAM-MD5, but proceeds wit PLAIN which
succeeded):

  05:04:16.672784 send: 'ehlo [127.0.0.1]\r\n'
  05:04:16.689710 reply: b'250-mx.sdf.org Hello 
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [xx.xxx.xxx.xx], pleased to meet you\r\n'
  05:04:16.689792 reply: b'250-ENHANCEDSTATUSCODES\r\n'
  05:04:16.689840 reply: b'250-PIPELINING\r\n'
  05:04:16.689877 reply: b'250-EXPN\r\n'
  05:04:16.689914 reply: b'250-VERB\r\n'
  05:04:16.689951 reply: b'250-8BITMIME\r\n'
  05:04:16.689987 reply: b'250-SIZE 2147483648\r\n'
  05:04:16.690023 reply: b'250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN\r\n'
  05:04:16.690060 reply: b'250-STARTTLS\r\n'
  05:04:16.690096 reply: b'250-DELIVERBY\r\n'
  05:04:16.690132 reply: b'250 HELP\r\n'
  05:04:16.690178 reply: retcode (250); Msg: b'mx.sdf.org Hello 
c-73-109-252-21.hsd1.wa.comcast.net [73.109.252.
  21], pleased to meet 
you\nENHANCEDSTATUSCODES\nPIPELINING\nEXPN\nVERB\n8BITMIME\nSIZE 
2147483648\nAUTH DIGEST-
  MD5 CRAM-MD5 PLAIN\nSTARTTLS\nDELIVERBY\nHELP'
  05:04:16.690386 send: 'AUTH CRAM-MD5\r\n'
  05:04:16.701705 reply: b'334 PDM3NTM_____________________________Lm9yZz4=\r\n'
  05:04:16.701776 reply: retcode (334); Msg: 
b'PDM3NT______________________________Lm9yZz4='
  05:04:16.701936 send: 'a2x1IDM3__________________________________c2ZjNk\r\n'
  05:04:16.713173 reply: b'535 5.7.0 authentication failed\r\n'
  05:04:16.713247 reply: retcode (535); Msg: b'5.7.0 authentication failed'
  05:04:16.713339 send: 'AUTH PLAIN ____\r\n'
  05:04:16.790840 reply: b'235 2.0.0 OK Authenticated\r\n'
  05:04:16.790938 reply: retcode (235); Msg: b'2.0.0 OK Authenticated'
  (235, b'2.0.0 OK Authenticated')
  05:04:16.791612 send: 'mail FROM:<foobar@sdf.org> size=5\r\n'
  05:04:16.810506 reply: b'250 2.1.0 <foobar@sdf.org>... Sender ok\r\n'
  05:04:16.810579 reply: retcode (250); Msg: b'2.1.0 <foobar@sdf.org>... Sender 
ok'
  05:04:16.810767 send: 'rcpt TO:<foobar@example.com>\r\n'
  05:04:16.821945 reply: b'250 2.1.5 <foobar@example.com>... Recipient ok\r\n'
  05:04:16.822010 reply: retcode (250); Msg: b'2.1.5 <foobar@example.com>... 
Recipient ok'
  05:04:16.822059 send: 'data\r\n'
  05:04:16.838365 reply: b'354 Enter mail, end with "." on a line by itself\r\n'
  05:04:16.838434 reply: retcode (354); Msg: b'Enter mail, end with "." on a 
line by itself'
  05:04:16.838474 data: (354, b'Enter mail, end with "." on a line by itself')
  05:04:16.838698 send: b'hello\r\n.\r\n'
  05:04:16.857556 reply: b'250 2.0.0 xxxxxxxxxxxxxx Message accepted for 
delivery\r\n'
  05:04:16.857629 reply: retcode (250); Msg: b'2.0.0 xxxxxxxxxxxxxx Message 
accepted for delivery'
  05:04:16.857669 data: (250, b'2.0.0 xxxxxxxxxxxxxx Message accepted for 
delivery')
  {}
  05:04:16.857730 send: 'QUIT\r\n'
  05:04:16.868917 reply: b'221 2.0.0 mx.sdf.org closing connection\r\n'
  05:04:16.868990 reply: retcode (221); Msg: b'2.0.0 mx.sdf.org closing 
connection'


Finally, as a work around, I manually set `smtpmail-auth-supported` to only
PLAIN. And this works.

  (setq smtpmail-auth-supported '(plain))



Below are Emacs Bug Report generated info:
-----------------------------------------

In GNU Emacs 27.1 (build 1, amd64-portbld-freebsd11.4)
System Description: 11.4-RELEASE-p6

Recent messages:
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Saving file /home/klu/News/drafts/drafts/3...
Wrote /home/klu/News/drafts/drafts/3
Deleting article /home/klu/News/drafts/drafts/1 in drafts...
current-kill: Kill ring is empty [4 times]
C-c k is undefined

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