sounds like a good reason to declare the whole thing Experimental,
or maybe Historic...
Not to me. Just because people can screw it up does not mean that
most will. We have massive experience with people screwing up SMTP,
MIME, etc. The fact that we have plenty of good implementations means
that we shouldn't be moving things to Experimental or Historic; to do
so would just be reactionary.
This has the potential to cause a lot more harm than the typical
screwed-up SMTP.... particularly if a huge portion of the installed
base is polluted by the irresponsibility of a major vendor.