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 notmuch-tag crm notmuch-lib notmuch-version notmuch-compat pcase hl-line message rmc dired dired-loaddefs rfc822 mml mailabbrev gmm-utils mailheader mm-view mml-smime mml-sec epa derived epg epg-config gnus-util rmail rmail-loaddefs mail-utils text-property-search time-date smime dig mm-decode mm-bodies mm-encode mailcap mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr vc-git server tempo ol-notmuch ol org-compat org-macs format-spec org-install vc vc-dispatcher circe-color-nicks color circe-chanop circe advice diff-mode lui-irc-colors irc make-tls-process tls gnutls puny lcs lui-format lui tracking shorten easy-mmode rx thingatpt paren help-mode flyspell ispell ring circe-compat bbdb-loaddefs preview-latex auto-loads tex-site finder-inf dpkg-dev-el info debian-el slime-autoloads package easymenu browse-url url-handlers url-parse auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs password-cache json subr-x map url-vars seq byte-opt gv bytecomp byte-compile cconv cl-loaddefs cl-lib tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock syntax facemenu font-core term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european ethiopic indian cyrillic chinese composite charscript charprop case-table epa-hook jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs button faces cus-face macroexp files text-properties overlay sha1 md5 base64 format env code-pages mule custom widget hashtable-print-readable backquote threads dbusbind inotify lcms2 dynamic-setting system-font-setting font-render-setting cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs) Memory information: ((conses 16 655778 54211) (symbols 48 31792 3) (strings 32 150840 22171) (string-bytes 1 4753274) (vectors 16 59825) (vector-slots 8 1381524 89918) (floats 8 1245 935) (intervals 56 18472 870) (buffers 1000 63))