The defcustom for mail-user-agent looks like
:type '(radio (function-item :tag "Message package"
:format "%t\n"
message-user-agent)
;; [...] snip
(function-item :tag "Message with full Gnus features"
:format "%t\n"
gnus-user-agent)
(function :tag "Other"))
This means that a symbol without a function definition cannot be set
(using customize) as mail-user-agent. This seem inconsistent with the
docstring of define-mail-user-agent,
SYMBOL can be any Lisp symbol. Its function definition and/or
value as a variable do not matter for this usage; we use only certain
properties on its property list, to encode the rest of the arguments.
and with existing mail user agents such as message-user-agent and
gnus-user-agent which don't have function definitions.
As far as I know, this defcustom is still enforcing that the symbol is a
function in emacs 28.x.
In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo
version 1.16.0)
of 2021-03-27, modified by Debian built on x86-conova-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)
Configured using:
'configure --build x86_64-linux-gnu --prefix=/usr
--sharedstatedir=/var/lib --libexecdir=/usr/lib
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --enable-libsystemd --with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
--with-sound=alsa --without-gconf --with-mailutils --build
x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
--libexecdir=/usr/lib --localstatedir=/var/lib
--infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
--with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
--with-sound=alsa --without-gconf --with-mailutils --with-cairo
--with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars
'CFLAGS=-g -O2
-ffile-prefix-map=/build/emacs-LlFm6W/emacs-27.1+1=. -fstack-protector-strong
-Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
-D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'
Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD
JSON PDUMPER LCMS2 GMP
Important settings:
value of $LANG: en_CA.UTF-8
locale-coding-system: utf-8-unix
Major mode: notmuch-show
Minor modes in effect:
shell-dirtrack-mode: t
tracking-mode: t
tooltip-mode: t
global-eldoc-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
menu-bar-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
buffer-read-only: t
line-number-mode: t
visual-line-mode: t
transient-mark-mode: t
Features:
(shadow sort company-oddmuse company-keywords company-etags etags
fileloop xref project company-gtags company-dabbrev-code company-dabbrev
company-files company-capf company-cmake company-xcode company-clang
company-semantic company-eclim company-template company-bbdb company
mail-extr emacsbug sendmail cursor-sensor smerge-mode diff mm-archive
shr-color org-clock org-duration view mule-util cal-china lunar solar
cal-dst cal-bahai cal-islam cal-hebrew holidays hol-loaddefs cal-iso
org-eldoc org-element avl-tree generator ol-eww eww mm-url url-queue
ol-rmail ol-mhe ol-irc ol-info ol-gnus nnir gnus-sum url url-proxy
url-privacy url-expand url-methods url-history shr url-cookie url-domsuf
url-util svg xml dom gnus-group gnus-undo gnus-start gnus-cloud nnimap
nnmail mail-source utf7 netrc nnoo gnus-spec gnus-int gnus-range
gnus-win gnus nnheader ol-docview doc-view jka-compr image-mode exif
ol-bibtex bibtex ol-bbdb ol-w3m face-remap org-agenda org-refile org
org-macro org-footnote org-pcomplete org-list org-faces org-entities
noutline outline org-version ob-dot ob-shell ob-emacs-lisp ob-python
python tramp-sh tramp tramp-loaddefs trampver tramp-integration files-x
tramp-compat shell pcomplete parse-time iso8601 ls-lisp ob ob-tangle
org-src ob-ref ob-lob ob-table ob-exp ob-comint comint ansi-color
ob-core ob-eval org-table org-keys org-loaddefs find-func edmacro kmacro
cl-extra notmuch notmuch-tree notmuch-jump notmuch-hello wid-edit
notmuch-show notmuch-print notmuch-crypto notmuch-mua notmuch-message
notmuch-draft notmuch-maildir-fcc notmuch-address notmuch-company
notmuch-parser notmuch-wash coolj notmuch-query goto-addr icalendar
diary-lib diary-loaddefs cal-menu calendar cal-loaddefs
