Given my druthers, I'd much rather see it be an extension only to the
MSA SUBMIT protocol and not one that is expected to be implemented in MTAs.
For the issue of efficient transfer where there is a bottleneck
(e.g. a dialup connection to an ISP), it might not be practical
to restrict it to the SUBMIT protocol; not all ISPs operate
servers on port 587.
if they're going to upgrade their servers to support the extension,
they may as well upgrade them to listen on port 587 at the same time.
this really needs to be an extension for SUBMIT only
Keith