Dave Crocker writes:
sender/from/reply-to were major innovations in RFC 733, considerably
pre-dating SMTP.
RFC 822 did not predate SMTP. There were many changes from 733 to 822.
Return-Path was added in 822. Why did Return-Path not replace Sender?
---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago