auto-responders and reply-to field

[Omar Thameen]

I just know it's true, but where does it say that auto-responders (e.g.,
vacation programs) must reply to the envelope from and certainly NOT to
the address in the reply-to field?

I can't find the language I'm looking for, and I've looked through RFCs
821, 822, 1891, and some others.

Omar