I believe that this is just a trivial mistake that simply needs fixing, rather than a large procedural issue, but am copying the IESG in case someone disagrees.
I believe the definition of STD 10 (SMTP, etc.) is incorrect. It now lists RFC 821 (SMTP) and RFC 1869 (SMTP extensions). If my memory is correct, it used to list RFC 974 (mail routing using MX records) as well. 821 is incomplete without 974 and support for 974 is mandatory (If I recall, RFC 1123 says so). So 974 should be included in STD 10. So, ideally, should the relevant sections of 1123, although that would set some new precedents.
This all gets straightened out with 2821ter, but that doesn't seem to be happening very quickly :-(
Could the definition of STD 10 be updated/corrected in the interim?
FWIW, I concur with John's analysis of what should be in STD 10. RCC 974 is an integral part of it; you cannot do contemporary email without it.