Undelivered Mail Returned to Sender

[MAILER-DAEMON@xxxxxxxxxxxxxxxxxxxxx (Mail Delivery System)]

This is the Proprietary program at host ZIXVPM01.CHVA-INT.ORG.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                        The Proprietary program

<rlockwoodi@xxxxxxxx>: host 172.16.0.16[172.16.0.16] said: 550 5.1.1 User
    unknown (in reply to RCPT TO command)

Reporting-MTA: dns; ZIXVPM01.CHVA-INT.ORG
X-Proprietary-Queue-ID: 7BDD415000A
X-Proprietary-Sender: rfc822; kernprof@oss.sgi.com
Arrival-Date: Sun, 20 Apr 2008 05:58:32 -0400 (EDT)

Final-Recipient: rfc822; rlockwoodi@chva.org
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Proprietary; host 172.16.0.16[172.16.0.16] said: 550 5.1.1
    User unknown (in reply to RCPT TO command)
> --- Begin Message ---
>
> --- End Message ---