Suppose we want to inspect the number of recipients for a message in the Headers view. We add the following to our configuration:
(defun show-number-of-recipients (msg) "Display the number of recipients for the message at point." (message "Number of recipients: %d" (+ (length (mu4e-message-field msg :to)) (length (mu4e-message-field msg :cc))))) ;; define 'N' (the first letter of the description) as the shortcut ;; the 't' argument to add-to-list puts it at the end of the list (add-to-list 'mu4e-headers-actions '("Number of recipients" . show-number-of-recipients) t)
After evaluating this, a N in the headers view shows the number of recipients for the message at point.