This is the NDR that a someone gets trying to send my client an email; they were kind enough to send me their NDR:
The following recipient(s) cannot be reached:
email@example.com on 4/20/2010 7:02 PM
The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
< THEM.THEIRDOMAIN.com #5.0.0 smtp; 5.4.7 - Delivery expired (message too old) [Default] 451-'4.7.0 Timeout waiting for client input' (delivery attempts: 24)>