Improved straw man for retry scenarios

Well, I guess I was right when I suggested that a separate document to
pass along our accumulated wisdom would be a good idea.

Easier to remember straw man:

  220 server

  EHLO foo
  250 whatever

  MAIL FROM:<s>               that's the Sender
  250 ok

  RCPT TO:<a>                 that's an Acceptable recipient
  250 ok

  RCPT TO:<d>                 that's a Delayed recipient
  450 try later

  RCPT TO:<r>                 that's a Rejected recipient
  550 no such user

  DATA
  blah blah
  .
  554 ugh

  QUIT
  220 server

Which, if any, of a, d, and r get retried?  Why or why not?  What if
the 554 were a 421 or 451?

In your answer, please consider RFC 2821 section 4.2.5, particularly
the suggestion to requeue the message in the antepenultimate paragraph
of that section.

R's,
John