That's the default message from Mailman when the admin rejects a message without doing a bunch of extra work, and the Mailman web UI makes it *really* easy to accidentally reject messages -- the "reject" button is only a few pixels away from the "accept" button, and there's *another* button that does "reject everything I didn't explicitly allow" right next to the "apply changes on this page" button. For an even slightly heavily trafficked mailing list, "reject all" is the only sane way to deal with Mailman's filtering UI.