Are you talking about old NDRs that have already been sent, or catching future ones?
The best you can do for old ones is to look through the logs where the sender = "<>". Sendmail doesn't store that data anywhere, it just delivers the NDR to the original sender.
As for future NDRs, I don't think Sendmail can forward them to some account by itself. There are many milters you can use to do this though. I use Mimedefang (
http://mimedefang.org) for spam filtering and you could easily add a bcc recipient to any message where the sender is blank.
