mail failed, returning to sender

["Arthur H. Smith" <arthurhsmith@compuserve.com>]

Message text written by INTERNET:MAILER-DAEMON@newman111.com
>|------------------------- Failed addresses follow: ---------------------|
 <arthur@newman111.com> ... unknown user
|------------------------- Message text follows: ------------------------|
Received: from mail.newman111.com[192.168.42.253] by mail.newman111.com
        with smtp
        id <m1216vA-0008X8C@mail.newman111.com>
        for <unknown>; Thu, 23 Dec 1999 12:00:40 -0700
Return-Path: <spitfires-owner@autox.team.net><

Why am I getting these things???   It was the second today.