If you have a message in rmail which is in MIME format with base64
encoding and consists only of a single text/plain part, and if you do
not display the "Content-Type" header (e.g. by having it in
rmail-ignored-headers), the message will get corrupted.  The problem is
this: under the circumstances above, rmail-edit-current-message allows
you to edit your view of the message (which is good, since you don't
want to edit the base64).  But when it goes to reencode the message, it
looks in the headers it gave you to edit and doesn't see the
Content-Type.  Later it does see the Content-Type in the original
headers, and the result is massive confusion.  In some circumstances it
corrupts only that message, but in others it corrupts your mail file by
merging this message with the one before.

To reproduce:

1. emacs -Q

2. Visit attached rmail-test file

3. M-x rmail-mode

4. Set variable rmail-ignored-headers to ignore "Content-Type", e.g., by
editing it in customization system.

5. Push "t" twice so that previous change takes effect.  Verify that
Content-Type is not displayed.

6. Push "e" to edit message.  Insert a character at the end.  C-c C-c to
finish.

7. Observe corrupted message on screen

I'm not sure how to reproduce the situation where it corrupts your mail
file, but it has happened to me several times.

I can provide a fix for this bug if we agree on the right strategy.

                                        Ken

In GNU Emacs 25.2.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2017-05-13 built on neptune
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:	Ubuntu 14.04.5 LTS

Configured features:
XPM JPEG TIFF GIF PNG SOUND NOTIFY ZLIB TOOLKIT_SCROLL_BARS LUCID X11

Important settings:
  value of $LC_ALL: C
  value of $LANG: en_US.UTF-8
  locale-coding-system: nil

Major mode: Lisp Interaction

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  line-number-mode: t

Recent messages:
Loading /home/kdo/emacs-init.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rmail rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
mail-prsvr mail-utils shell pcomplete comint ansi-color ring cl-macs cl
gv cl-loaddefs pcase cl-lib warnings time-date mule-util tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core 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 charscript case-table epa-hook jka-cmpr-hook help
simple abbrev 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
inotify dynamic-setting x-toolkit x multi-tty make-network-process
emacs)

Memory information:
((conses 16 95255 6032)
 (symbols 48 20728 0)
 (miscs 40 52 117)
 (strings 32 18042 4867)
 (string-bytes 1 512076)
 (vectors 16 13426)
 (vector-slots 8 443268 2725)
 (floats 8 169 6)
 (intervals 56 189 92)
 (buffers 976 18)
 (heap 1024 39178 767))