Suppose we want to inspect the number of recipients for a message in the 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.