(setf mm-verify-option 'always)
  (cl-pushnew "multipart/signed" gnus-buttonized-mime-types)

(defun mu4e-view-snarf-pgp-key (&optional msg)
  "Snarf the pgp key for the specified message."
  (interactive)
  (let* ((msg (or msg (mu4e-message-at-point)))
          (path (mu4e-message-field msg :path))
          (cmd (format "%s verify --verbose %s"
                 mu4e-mu-binary
                 (shell-quote-argument path)))
          (output (shell-command-to-string cmd)))
    (message "mu4e-view-snarf-pgp-key" ":msg=" msg ":path=" path ":cmd=" cmd ":output=" output)
    (let ((case-fold-search nil)
          (index 0))
      (while (string-match "finger-print[[:space:]]*: \\([A-F0-9]+\\)" output index)
        (let* ((cmd (format "%s --recv %s"
                            epg-gpg-program (match-string 1 output)))
               (output (shell-command-to-string cmd)))
          (setf index (match-end 0))
          (message output))))))

:hook
  (mu4e-compose-mode . (lambda () (mml-secure-sign)))