Select text in an Emacs buffer and send with M-x send-to-kindle-as-txt

send-to-kindle-as-txt.el

(defcustom send-to-kindle-from-email
  nil
  "Your own email address to send from via mu4e."
  :type 'string
  :group 'send-to-kindle)

(defcustom send-to-kindle-to-email
  nil
  "Your Kindle email address to send pdf to."
  :type 'string
  :group 'send-to-kindle)

(defun send-to-kindle-as-txt (review)
  (interactive "P")
  (unless send-to-kindle-from-email
    (setq send-to-kindle-from-email
          (read-string "From email address: ")))
  (unless send-to-kindle-to-email
    (setq send-to-kindle-to-email
          (read-string "To email address: ")))
  (let* ((content (string-trim (if (region-active-p)
                                   (buffer-substring (region-beginning) (region-end))
                                 (buffer-string))))
         (note-name (let ((name (string-trim (read-string "Note name: "))))
                      (if (string-empty-p name)
                          (nth
                           0 (string-split
                              (substring content 0 (min 40 (length content))) "\n"))
                        name)))
         (path (concat (temporary-file-directory) note-name))
         (txt (concat path ".txt"))
         (buffer (get-buffer-create (generate-new-buffer-name "*Email txt*"))))
    (with-temp-buffer
      (insert content)
      (write-file txt))
    (with-current-buffer buffer
      (erase-buffer)
      (message-mode)
      (insert
       (format
        "From: %s
To: %s
Subject: %s
--text follows this line--
<#multipart type=mixed>
<#part type=\"text/plain\" filename=\"%s\" disposition=attachment>
<#/part>
<#/multipart>"
        send-to-kindle-from-email
        send-to-kindle-to-email
        note-name txt))
      (unless review
        (message-send-and-exit)))
    (when review
      (switch-to-buffer buffer))))