In the Internet’s standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt (NDR), a (failed) Delivery Status Notification (DSN) message, a Non-Delivery Notification (NDN) or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another message about a delivery problem. The original message is said to have bounced.
When we need to disable NDR?
If your mail queues are getting full of NDRs from the postmaster to possible spammers, then disable ‘Allow delivery reports’ on the Property sheet:
Hub Transport\Remote Domains\Default Properties\Allow delivery reports.
Set-RemoteDomain -NDREnabled $false
For more Information about NDR; http://technet.microsoft.com/en-us/library/bb232118.aspx