bug#63332: 28.2.50; pop3.el does not support UTF-8 (RFC 6856)

[Benjamin Riefenstahl]

I run Gnus.  I get my mail from a POP3 server (courier-pop
1.0.16-3build3 on Ubuntu 22.4).  The server returns the following kind
of mail when there is a non-ASCII character somewhere in the headers of
the mail:

    From: Mail Delivery Subsystem <postmaster>
    Subject: Cannot display Unicode content
    Attachment: [2. message/global]...

    This E-mail message was determined to be Unicode-formatted
    but your E-mail reader does not support Unicode E-mail.

    Please use an E-mail reader that supports POP3 with UTF-8
    (see https://tools.ietf.org/html/rfc6856.html).

    This can also happen when the sender's E-mail program does not
    correctly format the sent message.

    The original message is included as a separate attachment
    so that it can be downloaded manually.

    [2. message/global]...

I can provide a full message, if needed.

Skimming RFC 6856, it looks like when the mail server provides the
capability "UTF8", the client should issue the command "UTF8" to
indicate that it is prepared to receive that encoding.

The attached patch tries to do just that.  I assume that there is not
any serious problem in the rest of the code with UTF-8.  The change does
not raise errors and it seems to work on test mails.  Please advise if
anything needs to be changed with the patch.

0001-Support-RFC-6856-bug-XXXXX.patch
Description: Text Data

Note 1: I assume similar code is needed in imap.el (RFC 6855), but I do
not have such a server, so I am not able to test a corresponding change.

Note 2: Using UTF-8 in the "Subject:" header does not work.  While the
subject is shown correctly in the summary, I get the usual bad display
in the mail itself.  But that should be a separate bug report, I think.

----

In GNU Emacs 28.2.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.16.0)
 of 2022-11-22 built on zenobia
Repository revision: 0636e1066bbafcd8219d9a8e09301c77f875d78b
Repository branch: emacs-28
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Group

Minor modes in effect:
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Load-path shadows:
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/home/benny/.emacs.d/elpa/soap-client-3.2.1/soap-client hides 
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