John Levine wrote:
RFC 821 came out in 1982. It makes only passing references to DNS,
because at the time the transition from HOSTS.TXT to the DNS had not
yet started. RFC 883, the first description of the DNS came out over
a year later in late 1983. It described the now abandoned MD and MF
records. According to RFCs 897 and 921, the transition to DNS started
in 1983, but HOSTS.TXT didn't go away until the end of 1985.
In January 1986, RFC 973 and 974 deprecated MD and MF records,
replaced them with MX, and defined the MX lookup with fallback to A.
While rereading 974, I note that it also recommends that clients do a
WKS lookup on each MX host to see if it actually supports SMTP and
discard the MX entry if it doesn't, but as far as I can tell, nobody
ever did that. Too bad.
So this means that SMTP had been in use for at least a year using
HOSTS.TXT, and then another couple of years using A, MD, and MF,
before MX came along, and I get the impression that MD and MF were
clunky enough that a most people just used the A record. Under the
circumstances, MX without fallback to A wouldn't have worked because
of the substantial installed base of mail servers using A records.
I think the above provides quite some insight into specifications
regarding the MX record, so I took the liberty of copying it in