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.