Add option to send a Delivery failed email
Add option to possibly send a delivery failed email to received emails. This would be really a great option for us to get rid of some senders.
I guess that you are thinking of this feature mainly for spam email. I agree that it may be useful for situations where you know that the sender's email address is legitimate; only the email addresses of most spammers are not, so it would not be effective in blocking such senders.
A better option, for spam at least, may be to send an error notification, on the server level, that would also include information for those maintaining the servers used in sending the email on how to stop their server(s) from being used for re-mailing purposes. That in addition to blacklists and RBL lists.