Can you post the complete message you get in response?
Example:
Your message did not reach some or all of the intended recipients.
Subject: hey so-and-so
Sent: 4/12/2006 1:30 AM
The following recipient(s) could not be reached:
so-and-so@x.com on 4/11/2006 5:35 PM
The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.
<x.x.com #5.1.1>