As implementers will need to read the normative RFC 2822 text, a simple reference to section 4.3 of RFC 2822 should suffice. Otherwise, the text should be as close as possible (i.e. identical) to the relevant 2822 text so as to avoid misinterpretation (Charles has said that an implementation might interpret "EST" as "unknown", which is certainly not the intent of RFC 2822).