The consensus on the basic issue of changing the wording of this
paragraph to include the word MUST:
The responsibility of an SMTP client is to transfer mail
messages to one or more SMTP servers, or report its
failure to do so.
In either case, a formal handoff of responsibility for
the message occurs: the protocol requires that a server
MUST accept responsibility for either delivering a
message or properly reporting the failure to do so.
However, several people raised issues with some ambiguities in the text.
I found Dave Crocker's description of the ambiguities to be the most
succinct. These ambiguities do exist whether the MUST is there or not,
so they pass my "chair filter", so should probably be dealt with.