What does "550 Sender verify failed" mean?

Here is a copy of what I am getting:


----- Original Message -----
From: "Mail Delivery System" <MAILER-DAEMON@mail.mydomain.com>
To: <william@virtualdomain.com>
Sent: Friday, March 09, 2007 1:02 PM
Subject: Undelivered Mail Returned to Sender> This is the Postfix program at host mail.mydomain.com.

I'm sorry to have to inform you that your message could not be
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 Postfix program

< macart@inkandpapermagic.com >: host inkandpapermagic.com[209.200.243.233]
said:
550-Verification failed for < william@virtualdomain.com > 550-unrouteable
mail
domain "virtualdomain.com" 550 Sender verify failed (in reply to RCPT TO
command)

Thanks,
Keith