(with-temp-buffer
       (insert all)
       (if indenting
-         (indent-rigidly (point-min) (point-max) (- depth)))
+         (indent-rigidly (point-min) (point-max) (- (* notmuch-show-indent-messages-width depth))))
       ;; Remove the original header.
       (goto-char (point-min))
       (re-search-forward "^$" (point-max) nil)
        (narrow-to-region part-beg part-end)
        (delete-region part-beg part-end)
        (apply #'notmuch-show-insert-bodypart-internal part-args)
-       (indent-rigidly part-beg part-end depth))
+       (indent-rigidly part-beg part-end (* notmuch-show-indent-messages-width depth)))
       (goto-char part-end)
       (delete-char 1)
       (notmuch-show-record-part-information (second part-args)