When generating cited messages for replay, override any existing
setting for `notmuch-show-indent-multipart' to ensure that no
indentation occurs.
                       ;; Don't omit long parts.
                       (notmuch-show-max-text-part-size 0)
                       ;; Insert headers for parts as appropriate for replying.
-                      (notmuch-show-insert-header-p-function notmuch-mua-reply-insert-header-p-function))
+                      (notmuch-show-insert-header-p-function notmuch-mua-reply-insert-header-p-function)
+                      ;; Don't indent multipart sub-parts.
+                      (notmuch-show-indent-multipart nil))
                    (notmuch-show-insert-body original (plist-get original :body) 0)
                    (buffer-substring-no-properties (point-min) (point-max)))))