graff (++) hit the nail on the head: Neither From: nor Reply To: are used to send 'Error-Information' Mails. The MTA uses the Return-Path: header. And this header can't be manipulated manualy, as it is set by the last MTA that treats your mail to the actual sender.

read more in the corresponding rfc 2076 (Common Internet Message Headers) Section 3.2 and rfc 821 (smtp) page 20ff..

regards,
tomte