So either put these clarifications into a separate draft and try to
advance it or start thinking carefully about what you are wishing
for.
I'd suggest a separate draft, since this is a potentially complex
topic where it'll be a challenge to document existing practice, much
less decide who's right when practice diverges.
Straw man:
220 server
EHLO foo
250 whatever
MAIL FROM:<a>
250 ok
RCPT TO:<b>
250 ok
RCPT TO:<c>
450 try later
RCPT TO:<d>
550 no such user
DATA
blah blah
.
554 ugh
QUIT
220 server
Which, if any, of b, c, and d get retried?
The message should be retried with recipient C only.
Why or why not?
Because the 554 could mean either that the message as a whole is unacceptable
or merely that no recipients were accepted.
What if the 554 were a 450?
Makes no difference.
Ned