However, if the extension only needs the support of the delivery agent,
then it can demand that MDAs participate by listing the extension as a
Delivery-Alert, and exclude the extension from the Transfer-Alert list. If
the MDA doesn't understand the extension, then delivery must fail (this
option is not available in SMTP today).
it's not clear why such an option would be useful or implemented - nobody
wants to cause a delivery to fail when the message has arrived at its
destination.
if we had the luxury of starting from scratch, I could see some benefit
of having the envelope protocol support a "delivery options" container
that wasn't examined until final delivery, perhaps even with criticality
flags. but experience with x.400 shows these things to be of limited
utility. so I don't think it really buys us very much, and I don't see
how whatever it would buy is likely to be worth the cost of transition.
that, and I don't even think a transition is feasible.
Keith
p.s. I realize that protocol engineers like to design new protocols. but I
really wish that people would think about how to make more reliable,
virus-proof, and spam-proof, and about how to solve other problems of
a similar severity, rather than trying to justify designing a new
protocol by promising new features of dubious value. Of course, these
are much harder problems to solve, and they involve not just protocol
design but also an understanding of operations and even politics,
which may explain why folks are reluctant to tackle them. However,
this is where we need to be applying our efforts.