Compose hooks (Mu4e 1.4.12 user manual)

6.4 Compose hooks

If you want to change some setting, or execute some custom action before message composition starts, you can define a hook function. mu4e offers two hooks:

mu4e-compose-pre-hook: this hook is run before composition starts; if you are composing a reply, forward a message, or edit an existing message, the variable mu4e-compose-parent-message points to the message being replied to, forwarded or edited, and you can use mu4e-message-field to get the value of various properties (and see Message functions).
mu4e-compose-mode-hook: this hook is run just before composition starts, when the whole buffer has already been set up. This is a good place for editing-related settings. mu4e-compose-parent-message (see above) is also at your disposal.

Let’s look at some examples. First, suppose we want to set the From:-address for a reply message based on the receiver of the original:

;; 1) messages to me@foo.example.com should be replied with From:me@foo.example.com
;; 2) messages to me@bar.example.com should be replied with From:me@bar.example.com
;; 3) all other mail should use From:me@cuux.example.com
(add-hook 'mu4e-compose-pre-hook
  (defun my-set-from-address ()
    "Set the From address based on the To address of the original."
    (let ((msg mu4e-compose-parent-message)) ;; msg is shorter...
      (when msg
	(setq user-mail-address
	  (cond
	    ((mu4e-message-contact-field-matches msg :to "me@foo.example.com")
	      "me@foo.example.com")
	    ((mu4e-message-contact-field-matches msg :to "me@bar.example.com")
	      "me@bar.example.com")
	    (t "me@cuux.example.com")))))))

Secondly, as mentioned, mu4e-compose-mode-hook is especially useful for editing-related settings. For example:

(add-hook 'mu4e-compose-mode-hook
  (defun my-do-compose-stuff ()
    "My settings for message composition."
    (set-fill-column 72)
    (flyspell-mode)))

This hook is also useful for adding headers or changing headers, since the message is fully formed when this hook runs. For example, to add a Bcc:-header, you could add something like the following, using message-add-header from message-mode.

(add-hook 'mu4e-compose-mode-hook
  (defun my-add-bcc ()
    "Add a Bcc: header."
    (save-excursion (message-add-header "Bcc: me@example.com\n"))))

Or to something context-specific:

(add-hook 'mu4e-compose-mode-hook
  (lambda()
    (let* ((ctx (mu4e-context-current))
	    (name (if ctx (mu4e-context-name ctx))))
      (when name
	(cond
	  ((string= name "account1")
            (save-excursion (message-add-header "Bcc: account1@example.com\n")))
	  ((string= name "account2")
            (save-excursion (message-add-header "Bcc: account2@example.com\n"))))))))

For a more general discussion about extending mu4e, see Extending mu4e.