bug#26307: 25.1; smtpmail hard-coded auth mechanism order does not allow coping with misbehaving servers

[Marco Maggi]

In GNU  Emacs 25.1.1 (x86_64-slackware-linux-gnu, GTK+  Version 2.24.30)
the library "smtpmail.el" contains the hard-coded list of authentication
mechanisms to try in sequence:

   (defconst smtpmail-auth-supported '(cram-md5 plain login)
     "List of supported SMTP AUTH mechanisms.
   The list is in preference order.")

it would be useful  to have a public API mechanism  to change such order
in case the server misbehaves when using the cram-m5 method.

  I am  customer of  a mail  service using a  server that  advertises to
support  the cram-md5,  plain and  login  methods but  then when  trying
cram-md5 the authentication fails; the login method succeeds.

  Just changing the above code to:

   (defconst smtpmail-auth-supported '(login cram-md5 plain)

and recompiling fixes my problems.

  It is hard  to contact the service administrators and  having them fix
the problem, so a workaround is useful.

TIA
-- 
Marco Maggi