Undelivered Mail Returned to Sender

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

This is the Postfix program at host teresa.winkowski.pl.

I'm sorry to have to inform you that the message returned
below could not be delivered to one or more destinations.

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 message returned below.

                        The Postfix program

<robert@xxxxxxxxxxxxxxxx>: host 192.168.5.100[192.168.5.100] said: 550
    <robert@xxxxxxxxxxxxxxxx>: User unknown in virtual mailbox table (in reply
    to RCPT TO command)

Reporting-MTA: dns; teresa.winkowski.pl
Arrival-Date: Sun, 20 Apr 2008 11:17:01 +0200 (CEST)

Final-Recipient: rfc822; robert@winkowski.com.pl
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; host 192.168.5.100[192.168.5.100] said: 550
    <robert@winkowski.com.pl>: User unknown in virtual mailbox table (in reply
    to RCPT TO command)
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