As Ned said, this is a major deviation from what an envelope
sender address used to be for some decades, and it's IMO not
*obvious* how it works in conjunction with RFC 3834 + SIEVE.
How far back do you want to go? People have been using subaddresses
like user+blah(_at_)domain since 1991. Mailing lists have been using VERP
schemes to encode per-message and per-recipient information into the
bounce address at least since 1998.
The advice in 3834 to send vacation responses to the bounce address
was already dubious in 2004, since by then there was lots of list
management software that interpreted anything sent back to a VERP
address as a bounce. (Yes, I know that a sensible vacation program
that followed all the advice in 3834 wouldn't respond to list mail at
all, but as we all know, life is not that tidy.)
R's,
John