Negotiation can mean that A and B receive different messages, both stating
(in to/cc) that the other received the one they didn't. Sounds very fishy
to my ears.
That's an interesting point. However it's always been possible for
recipients listed in the to/cc fields to receive different versions
of the message, for some of those listed to not receive the message
at all, or for additional recipients to receive the message.
Keith