mu4e - Changing existing headers

Question

I'm using contexts within mu4e, and I know how to add custom headers via the mu4e-header-info-custom list.

What I'd like to do, however, is to alter existing headers when replying or composing within certain contexts.

Here's one example: if I reply to a message that matches one of these special contexts, I'd like the To: header to be automatically changed to a different address, or to add additional addresses to the To: line.

Suppose the sender is sender@domain.com. If I reply to the message, I'd like the To: line to automatically be changed to:

To: newuser@domain.com

... or even perhaps to ...

To: sender@domain.com, newuser1@domain.com, newuser2@domain.com

I might want to automatically change other existing headers, as well, such as Subject:.

Is there any way to do this in mu4e?

Answer 1

I will not take credit for this answer. cpbotha wrote a very informative blog post on this very topic. There are a few functions that need to be copied into your init.el or .emacs file, one for checking the recipient of the email, then executing a function that updates the user-full-name, user-mail-address, and updates the signature. This is a gist for you, but refer to the original blog for more useful configuration ideas.

(defun get-string-from-file (filePath)
  "Return FILEPATH's file content."
  (with-temp-buffer
    (insert-file-contents filePath)
    (buffer-string)))

(defun my-mu4e-personal()
  (interactive)
  (message "personal mail account")
  (setq user-full-name "MyFirst MyLast"
    user-mail-address "my.email@test.com"
    mu4e-compose-signature (get-string-from-file "~/.signature")))

(defun my-mu4e-work()
  (interactive)
  (message "work mail account")
  (setq user-full-name "MyFirst MyLast"
    user-mail-address "my.email@work-test.com"
    mu4e-compose-signature (get-string-from-file "~/.signature-work")))

;; when you reply to a message, use the identity that the mail was sent to
;; the cpbotha variation -- function that checks to, cc and bcc fields
(defun cpb-mu4e-is-message-to (msg rx)
  "Check if to, cc or bcc field in MSG has any address in RX."
  (or (mu4e-message-contact-field-matches msg :to rx)
      (mu4e-message-contact-field-matches msg :cc rx)
      (mu4e-message-contact-field-matches msg :bcc rx)))

;; we only do something if we recognize something (i.e. no stupid default)
(add-hook 'mu4e-compose-pre-hook
  (defun my-set-from-address ()
    "Set current identity based on to, cc, bcc of original."
      (let ((msg mu4e-compose-parent-message)) ;; msg is shorter...
        (if msg
          (cond
           ((cpb-mu4e-is-message-to msg "my.email@test.com")
            (my-mu4e-personal))
           ((cpb-mu4e-is-message-to msg "my.email@work-test.com")
            (my-mu4e-work)))))))