If we forget about portability adding "Date" and "Message-ID" can be
reduced to this:

Index: send.c
===================================================================
RCS file: /cvs/src/usr.bin/mail/send.c,v
diff -u -p -r1.26 send.c
--- send.c	8 Mar 2023 04:43:11 -0000	1.26
+++ send.c	5 Aug 2024 11:54:18 -0000
@@ -516,15 +516,29 @@ puthead(struct header *hp, FILE *fo, int
 {
 	int gotcha;
 	char *from;
+	time_t t = time(NULL);
+	struct tm tm = *localtime(&t);
+	char date[32];
+	int hs, ts;
+	char hostname[1024];
+
+	hs = gethostname(hostname, 1023);
+	ts = strftime(date, sizeof(date), "%a, %d %b %Y %T %z", &tm);
 
 	gotcha = 0;
 	from = hp->h_from ? hp->h_from : value("from");
 	if (from != NULL)
 		fprintf(fo, "From: %s\n", from), gotcha++;
+	if (fo != stdout && ts > 0)
+		fprintf(fo, "Date: %s\n", date), gotcha++;
 	if (hp->h_to != NULL && w & GTO)
 		fmt("To:", hp->h_to, fo, w&GCOMMA), gotcha++;
 	if (hp->h_subject != NULL && w & GSUBJECT)
 		fprintf(fo, "Subject: %s\n", hp->h_subject), gotcha++;
+	if (fo != stdout && hs == 0)
+		fprintf(fo, "Message-ID: <%d%02d%02d.%02d%02d%02d@%s>\n",
+		    tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday, tm.tm_hour,
+		    tm.tm_min, tm.tm_sec, hostname), gotcha++;
 	if (hp->h_cc != NULL && w & GCC)
 		fmt("Cc:", hp->h_cc, fo, w&GCOMMA), gotcha++;
 	if (hp->h_bcc != NULL && w & GBCC)