A new mbox handling in mailutils-3.11.1 prevents mail from saving changes because of a permission problem. The same bug is reported for Debian here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=980042 A fix commited upstream can be found here: http://git.savannah.gnu.org/cgit/mailutils.git/commit/?id=07ee21c2db558106c8537600698db75e51ac1090
fixed in mailutils-3.12. Closing