bug#36937: 26.1; Sending mails doesn't set From: correctly

[Jose A. Ortega Ruiz]

This problem started happening in builds from master around 4 days ago:

- start emacs -Q
- M-x mail (or any other form of sending email)
- Set a From: value different from user@localhostname
- Send mail using local sendmail

The mail is sent with a from header seen by postfix (i don't know about
other MTAs) as user@localhost, instead of the From: i set in the
headers.  By user@localhost i mean the unix username running emacs and
the hostname as returned by hostname(1).  It seems emacs is just
ignoring the value of From:, or setting it in a way postfix doesn't
understand.

I know almost for sure it's an emacs problem because emacs 26 or an
earlier version of emacs-snapshot is working without problems.

Thanks!


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-08-03, unofficial emacs-snapshot build: http://emacs.secretsauce.net 
built on lilbastard
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Sending...
Save this mail sending choice? (y or n) n
Sending...done

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 --with-pop=yes
 
--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/27.0.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.0.50/site-lisp:/usr/share/emacs/site-lisp
 --build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes
 
--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/27.0.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.0.50/site-lisp:/usr/share/emacs/site-lisp
 --with-x=yes --with-x-toolkit=lucid --with-toolkit-scroll-bars
 --without-gconf --without-gsettings 'CFLAGS=-g -O2
 
-fdebug-prefix-map=/build/emacs-snapshot-eSXVQs/emacs-snapshot-20190803+emacs-26.1-6779-g5ae1191e5b0=.
 -fstack-protector-strong
 -Wformat -Werror=format-security -Wall -fno-omit-frame-pointer'
 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG SOUND GPM DBUS GLIB NOTIFY INOTIFY ACL
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM THREADS PDUMPER LCMS2 GMP

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
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 menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame 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 minibuffer 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
font-render-setting x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 44987 9529)
 (symbols 48 6203 1)
 (strings 32 16184 1920)
 (string-bytes 1 513447)
 (vectors 16 9265)
 (vector-slots 8 129612 10712)
 (floats 8 24 26)
 (intervals 56 271 0)
 (buffers 992 13))