How does the 'discard' action of RFC5617 (DKIM ADSP) fit into this
picture? Although RFC5321 and RFC5617 operate on different protocol
layers, in real life the two are often intertwined by way of MTA milter
callouts during SMTP DATA phase.
For people who choose to implement ADSP (and you know how I feel about
that), I think it's fair to say that it operates at a layer above SMTP.
If you transported your mail by uucp, DKIM and ADSP would work exactly
the same way as they do if it's transported by SMTP.