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.
First, RFC 2746 is quite clear: Port 25 MAY be used for SUBMIT if for whatever
reason 587 cannot be used.
More generally, what port something is run on is largely irrelevant; the
important issues are service and protocol semantics.
Of course this doesn't mean someone won't attempt to use SUBMIT-specific
extensions in regular SMTP. But just because some clueless ISP engages in
egregiously bad practice is no reason to break our protocol specifications. I
am dubious about the value of such an extension in any case for all the reason
I've previsouly given; I would vehemently oppose making it part of regular
SMTP.
Ned