If the sender is concerned about optimizing for each recipient, they can
get that effect by reducing to a single RCPT-TO per DATA.
if several recipients have similar capabliities, sending a separate
copy of each message hardly seems like an optimization.
So it is not as pretty as the separate command, but it permits roughly the
same mode of operation.
overloading RCPT is simply unacceptable. It would be a Bad Idea for
any SMTP extension, but this proposal pre-empts RCPT error reporting
for a very specific application.
Keith