Another issue you have to deal with here is that in the SMTP dialog between client and server, the MAIL FROM envelope address is sent before the RCPT TO envelope address, so any dependency must necessarily involve buffering the message and determining the proper envelope addresses based on the body contents. Thus, the use of a simple script and a sendmail workalike replacement makes sense. My guess would be that there's probably no way around using some sort of intermediate process to do the rewriting and the proper management of the envelope addresses.