Problems with new smtpmail

[Óscar Fuentes]

After building emacs with the sources from July 1, I can't send e-mails
anymore.

NEWS contain some info, but insufficient.

This is my setup:

(custom-set-variables
 ...
 '(message-send-mail-function (quote smtpmail-send-it))
 '(send-mail-function (quote smtpmail-send-it))
 '(smtpmail-default-smtp-server "smtp.server.net")
...
 )

(load-library "smtpmail")

(setq smtpmail-auth-credentials  ; or use ~/.authinfo
       '(("smtp.server.net" 25 "mylogin" "mypass")))

First, smtpmail-default-smtp-server is ignored. I have to set it with
setq before (load-library "smtpmail"). The documentation for
smtpmail-default-smtp-server says it is only effective if setted before
smtpmail is loaded, but it would be nice to mention that on NEWS too for
the benefit of those who used Emacs' standard customization interface
for setting the variable.

Second, NEWS says that smtpmail-auth-credentials no longer exists, but

".. if it were a direct list of user names and passwords, you will be
prompted for the user name and the password instead, and they will then
be saved to ~/.authinfo."

The above makes me think that smtpmail does something with
smtpmail-auth-credentials if it is assigned, which is contrary to my
experience, but maybe the questions are made on a stage that I didn't
reach, because the problem mentioned below.

So I go ahead and add

machine smtp.server.net login mylogin password mypass

to ~/.authinfo.

but sending mail fails with this:

smtpmail-send-it: Sending failed: Server supports STARTTLS, but Emacs
does not have support for it

Emacs worked fine for sending mail through that smtp server for
years. Now it seems that something is missing. What's the problem?

In any case, I find the information about the smtpmail change in NEWS
very sparse. Please note that most people is not an expert on mail
protocols and configured Emacs following recipes, possibly long time
ago. It is desirable to provide thorough documentation for updating the
most usual configurations to the new requirements.