This strongly implies that it cannot be carried via non-ESMTP
conformant SMTP MTA relays.
This is where I have a big problem. If we are looking at the ESMTP work
as a mechanism that will help convey more meaningful reports in certain
situations, then great. If on the other hand, we are mandating that a
message travel from originator to recipient *only* via ESMTP links, then
Just an observation and historical review, not a suggestion...
We've been down this path, and the "force it into the header if you
can't keep it in the envelope" consequence several times. The last
times, the conclusion was that it is a dead end if you want the ack
request to be anything but discretionary advice to whatever manages to
pick it up.
Otherwise, the reasoning went, you've got to have explicit handoffs to
transfer agents that are willing to accept responsibility for
acknowledgements. If an MTA is encountered that isn't (SMTP-only,
ESMTP without ack extension, gateway to environment that doesn't accept
Acks,...) then the MTA deciding to hand off to it presumably sends the
ack right then, indicating that delivery couldn't be made via an
Ack-ing path past that point.
Or, if one wanted to complexify this a bit, one could also move
originator instructions along in the envelope about whether to
forward-without-endpoint-ack or bounce the message.