Add another hook to "live encode" format=flow

based off some code at:

https://emacs.stackexchange.com/questions/19296/retooling-fill-paragraph-to-append-trailing-spaces

README.org

@@ -26,5 +26,23 @@ then add the following to your =.emacs= file:
 (add-hook 'message-mode-hook 'messages-are-flowing-use-and-mark-hard-newlines)
 #+END_SRC
 
+This package also supports inserting spaces for soft newlines via the
+following:
+
+#+BEGIN_SRC emacs-lisp
+(add-hook 'message-mode-hook 'messages-are-flowing-enhance-fill-newline)
+#+END_SRC
+
+NOTE: the modes need to be specified when to use the enhanced
+fill-newline:
+
+#+BEGIN_SRC emacs-lisp
+(use-package messages-are-flowing
+  :custom (messages-are-flowing-enhance-fill-newline-modes 'message-mode)
+)
+#+END_SRC
+
+This can be useful if using a mailer that does not post-encode the
+text (ie: mutt).
 
 #+STARTUP: showall

messages-are-flowing.el

@@ -42,6 +42,14 @@
 ;;
 ;; (with-eval-after-load "message"
 ;;   (add-hook 'message-mode-hook 'messages-are-flowing-use-and-mark-hard-newlines))
+;;
+;; If your mail program does not encode the flowed format, the following
+;; can be used to convert soft newlines to spaces:
+;;
+;; (add-hook 'message-mode-hook 'messages-are-flowing-enhance-fill-newline)
+;;
+;; Also note that messages-are-flowing-enhance-fill-newline-modes will need to
+;; be set with the modes the enhanced fill-newline should be used in
 
 ;;; Code:
 
@@ -55,6 +63,23 @@ device, such that messages remain readable on narrow displays."
   (use-hard-newlines)
   (add-hook 'after-change-functions 'messages-are-flowing--mark-hard-newlines nil t))
 
+;;;###autoload
+(defun messages-are-flowing-enhance-fill-newline ()
+  "Function to enhance fill-newline, only for certain modes"
+	(advice-add 'fill-newline :around #'messages-are-flowing--fill-newline-wrapper)
+	(turn-off-auto-fill)
+  )
+
+(defgroup messages-are-flowing nil
+  "Utilities for working with format=flowed messages"
+  )
+
+(defcustom messages-are-flowing-enhance-fill-newline-modes nil
+  "Modes to apply to when messages-are-flowing-enhance-fille-newline is in use"
+  :type 'function
+  :group 'messages-are-flowing)
+
+
 (defun messages-are-flowing--mark-hard-newlines (beg end &rest _ignore)
   "Visibly mark hard newlines between BEG and END.
 For each hard newline, add a display property that makes it visible.
@@ -70,5 +95,54 @@ For each soft newline, remove any display property."
             (add-text-properties pos (1+ pos) (list 'display (copy-sequence "⏎\n")))
           (remove-text-properties pos (1+ pos) '(display nil)))))))
 
+
+;; fill-newline that puts a space for soft newlines
+;; Based on https://emacs.stackexchange.com/questions/19296/retooling-fill-paragraph-to-append-trailing-spaces
+(defun messages-are-flowing--fill-newline ()
+  "Custom version of fill-newline to behave 3676ishly"
+
+  ;; Replace whitespace here with one newline, then
+  ;; indent to left margin.
+  (skip-chars-backward " \t")
+  (insert ?\s)
+  (insert ?\n)
+  ;; Give newline the properties of the space(s) it replaces
+  (set-text-properties (1- (point)) (point)
+					   (fill-text-properties-at (point)))
+  (and (looking-at "\\( [ \t]*\\)\\(\\c|\\)?")
+	   (or (aref (char-category-set (or (char-before (1- (point))) ?\000)) ?|)
+		   (match-end 2))
+	   ;; When refilling later on, this newline would normally not be replaced
+	   ;; by a space, so we need to mark it specially to re-install the space
+	   ;; when we unfill.
+	   (put-text-property (1- (point)) (point) 'fill-space (match-string 1)))
+  ;; If we don't want breaks in invisible text, don't insert
+  ;; an invisible newline.
+  (if fill-nobreak-invisible
+	  (remove-text-properties (1- (point)) (point)
+							  '(invisible t)))
+  (if (or fill-prefix
+		  (not fill-indent-according-to-mode))
+	  (fill-indent-to-left-margin)
+	(indent-according-to-mode))
+  ;; Insert the fill prefix after indentation.
+  (and fill-prefix (not (equal fill-prefix ""))
+	   ;; Markers that were after the whitespace are now at point: insert
+	   ;; before them so they don't get stuck before the prefix.
+	   (insert-before-markers-and-inherit fill-prefix)))
+
+(defun messages-are-flowing--fill-newline-wrapper (orig-fun &rest args)
+  "Call [messages-are-flowing--]fill-newline depending on mode"
+
+  ;; NOTE: this is done so that it doesn't affect other modes
+  (if (equal major-mode messages-are-flowing-enhance-fill-newline-modes)
+	  (progn ;; allow multiple statements
+		 ;; Alter fill-newline
+		 (messages-are-flowing--fill-newline)
+		)
+	(apply orig-fun args)
+	)
+  )
+
 (provide 'messages-are-flowing)
 ;;; messages-are-flowing.el ends here