Not sure I understand. Is your request satisfied, and are you happy with it? Or not, and if so, are you going to improve the proposal, or do you expect us to do so?
The order depends on how the sender put the lines. Try awk '
BEGIN {for (MX=i=split ("From To: Subject: Message-ID: Date:", KEYS); i>0; i--) VALS[KEYS[i]]
}