- STD 72 is RFC 6409 and covers "mail submission". That's what ordinary users like you and me do when they send e-mail using their ISP or another ESP such as Gmail, GMX, or Hotmail.
- STD 71 is RFC 6152 and covers "8BITMIME". That's what everybody uses since about 1993 outside of pure US-ASCII 7bit e-mail.
Oddly YAM didn't finish the work on a 5321bis and a 5322bis. That leaves us with STD 10 (SMTP) and STD 11 (message format) in limbo, unless you still consider RFCs 821 and 822 (vintage 1982) as the ultimate truth in IETF e-mail standards.
For SPF it's quite interesting to look at the "reverse path" fine print of RFC 821, e.g.:
- The first host in the <reverse-path> should be the host sending this command. (RFC 821 about MAIL FROM).
- In any case, the SMTP adds its own identifier to the reverse-path. (RFC 821 about Relaying).
Sadly RFC 4408 specifying SPF also claims that this RFC 821 feature is archaic. Of course today nobody uses reverse and forward paths as designed about thirty years ago for e-mail routing, but for SPF the MAIL FROM address is still supposed to indicate the responsible party for technical problems with an e-mail, also known as bounce address. And e-mails forwarded to a third party need a new envelope sender address, otherwise SPF checks at this third party will fail, because IP-addresses authorized to send e-mail from the first party won't match IPs used by the forwarding second party. That's as simple as it can be, ignoring the minor detail that RFC 1123 broke it about twenty years ago.
By the way, the work on 4408bis is supposed to start really soon now, but my confidence in the IETF is somewhat limited.