Go with 1A and 1B as MANDATORY. Those have always made sense. Don't
have anything OPTIONAL; if their importance becomes obvious later then
they can be added as extensions.
I don't see how D (email body modification) "falls back" to A (command
modification), but as long as it can be done, I don't care what letter
gets assigned to it.
I'd not thought that the OPES processor would be maintaining queues,
but if implementors think that's the way they want to do it, that's
fine. If the queues were maintained by the callout server, then
the difference between operating on SMTP vs. stored data would
be irrelevant.
Hilarie