Arnt Gulbrandsen wrote:
2821 4.5.5 lists some types for which you should use the null
path. It doesn't forbid using <> for types not on its list.
3461 5.2.2 (d), 3834 3.3 and 5230 5.1 add to the list, and I
would not be surprised if other RFCs add more and code does
even more.
2821bis explicitly mentions RFCs 3461 and 3798 in this section.
RFC 5230 is an application of RFC 3834. There is no reference
in 2821bis. I recall that John discussed it with Keith, maybe
that should be fixed, adding an RFC 3834 reference in 4.5.5 (?)
4.5.5 says that MAIL FROM: SHOULD be not empty in scenarios not
covered by standards-track RFCs mandating an empty reverse-path.
^^^^^^^^^^^^^^^^^^^^
IMHO exploring the far end of SHOULD on the border to MUST.
Frank