Which mail client in Emacs can schedule mail sending to every specific mail?

Question

In office, I need to send one mail to boss every day, every week, and every month. Usually I composed those mails every afternoon and leave them in a * send mail * buffer, and send them when I prepare to go home. Sometimes I forget to send the mail even if I finished composing them. The next day I found the * send mail * buffer and BANG I felt very upset.

AFAIK, there are several ways in Emacs to compose a mail and send a mail. Are there any methods to schedule a specific mail sending in a specific time, for example, at 6:00pm later that day? If not, how to write a function so that I can run the function for a specific mail and let them be sent at the given time?

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EDITED:

This question is something different from the similar Gnus question. I want to set a different schedule for each mail to be sent.

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Here I provide some pseudocode that come to my mind, but this does not works. Can anyone give me some advices? Thank you!

;; one pseudocode:
(require 'midnight)
(defun schedule-send-mail-in-current-buffer ()
  (interactive ???)
  (prompt-and-get buffer-name)
  (prompt-and-get send-time) ;; like 19:00pm
  (rename-this-buffer buffer-name)
  (bury-buffer (get-buffer buffer-name))
  ;; how to set this buffer to not-killable?
  (midnight-delay-set 'midnight-delay "19:00pm")
  (add-hook 'midnight-hook (lambda ()
                             (with-current-buffer buffer-name
                               (call-interactively 'message-send)))))

;; some similar psuedocode
(defun schedule-send-mail-in-current-buffer ()
  (interactive ???)
  ;; days hour minute after now
  (prompt-and-get buffer-name)
  (prompt-and-get minute)
  (prompt-and-get hour)
  (prompt-and-get days)

  (rename-this-buffer buffer-name)
  (bury-buffer (get-buffer buffer-name))
  ;; calc schedule-time
  (let ((scheduled-time (build-date-time minute hour days)))
    ;; if setq timer is outside defun, it works. Why cannot it work in a defun?
    (setq monthcheck-timer
          (run-at-time
           (encode-time (get-second scheduled-time)
                        (get-minute scheduled-time)
                        (get-hour scheduled-time)
                        (get-day scheduled-time)
                        (get-month scheduled-time)
                        (get-year scheduled-time))
           nil
           (lambda ()
             (with-current-buffer buffer-name
               (call-interactively 'message-send)))))))

Answer 1

There is an accepted answer to a similar question.

Extracted information: yes, you can if you use Gnus.
See above mentioned answer for instructions.