There has always been a many to 1 relationship between representations
of the "sender mailbox" and what might be considered to be the
"canonical" representation. Consider aliases. Consider subaddresses.
Consider case differences. Consider limited-use aliases.
Which one goes into the 821.MAILFROM? Has it ever mattered? Does it
matter if the MTA generates one "on the fly"?
Which is "the sender mailbox"? Just the canonical representation, or all
Alessandro Vesely wrote:
Section 18.104.22.168 additionally states that "The reverse-path consists of
the sender mailbox", not a variation thereof. That wording apparently
bans using time-varying tags, unless we reinterpret BATV as a
redistribution service for ephemeral ad-hoc lists, in the sense of
section 3.9.2 (but beware poor subscription policies.) A rather
cumbersome way to standardize things.