Here's a thought I just thunk, folks: some scum, apparently in eastern Europe, has harvested my email, and is using it in the Reply-To: in its spamming efforts. Now, I realize that some mails go out from noreply, but other than that, is there a good reason why a mailserver would not be configured to send delivery failure to *both* Reply-To and From?
You don't want to send rejects to more than one address 'cos you then have a simple message multiplier; send one message, generate two bounces; the mail server will be doubling the back-scatter problem!
Anyway, the SMTP server should send the delivery failure to the envelope address, which may be different to both the From and Reply-To addresses.