(with-eval-after-load 'notmuch
  (defun my-notmuch-show-browse-url-or-toggle-message ()
    (interactive)
    (if (thing-at-point 'url)
        (goto-address-at-point)
      (notmuch-show-toggle-message))))

(with-eval-after-load 'notmuch
  (keymap-set notmuch-show-mode-map
              "RET"
              #'my-notmuch-show-browse-url-or-toggle-message))