because there is no way to save the list of inbox files in an
mbox-format file.
+Also, whereas with Babyl format M-x find-file would switch to Rmail
+mode, with mbox format this is no longer the case (there being no way
+to add an "-*- rmail-*-" cookie to an mbox file).
+
If you have written any extensions to Rmail, they are likely to need
updating. Conceptually, the Rmail buffer that you see is no longer
just a narrowed portion of the whole. So you cannot access the whole