(with-eval-after-load 'notmuch

  (defun my-notmuch-show-toggle-unread ()
    "Toggle unread tag and advance to next message."
    (interactive)
    (notmuch-show-tag
     (list (if (member "unread" (notmuch-show-get-tags))
               "-unread" "+unread")))
    (notmuch-show-next-message))

  (defun my-notmuch-show-toggle-spam ()
    "Toggle spam tag and advance to next message."
    (interactive)
    (notmuch-show-tag
     (if (member "spam" (notmuch-show-get-tags))
         '("-spam" "+unread") '("+spam" "-unread")))
    (notmuch-show-next-message))

  (keymap-set notmuch-show-mode-map "k" #'my-notmuch-show-toggle-unread)
  (keymap-set notmuch-show-mode-map "K" #'my-notmuch-show-toggle-spam))