Вот обертка для mail-mail, которая запрашивает строки 'to' и 'subject':
(defun my-message-mail-region (b e to subject)
"Send the current region in an email"
(interactive "r\nsRecipient: \nsSubject: ")
(let ((orig-buffer (current-buffer)))
(message-mail to subject)
(message-goto-body)
(insert (save-excursion (set-buffer orig-buffer)
(buffer-substring-no-properties b e)))
(message-send-and-exit)))