Getting lots of Undeliverable: Returned mail: see transcript for details

[Billy A. Pumphrey]

Since I upgraded to near the latest MailScanner, I am getting a lot of
these.

Your message did not reach some or all of the intended recipients.

      Subject:	Returned mail: see transcript for details
      Sent:	4/15/2006 9:32 PM

The following recipient(s) could not be reached:

      jelki at selena.net.ua on 4/15/2006 9:32 PM
            The message could not be delivered because the recipient's
destination email system is unknown or invalid. Please check the address
and try again, or contact your system administrator to verify
connectivity to the email system of the recipient.
            < WoodenMS2.woodmaclaw.local #5.1.2>	

Any idea why this is happening?  I figure that is is DNS related or
something.

Thank you