MX Record - History of Fallback To A

History of Fallback To A

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 experimental and little used MD and MF records. According to RFC 897 and RFC 921, the transition to DNS started in 1983, but HOSTS.TXT wasn't scheduled to go away until the end of 1985 and wasn't totally phased out until the late 1990s.

In January 1986, RFC 973 and RFC 974 deprecated the MD and MF records, replaced them with MX, and defined the MX lookup with fallback to A. RFC 974 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. However, RFC 1123 changed this to say that WKS should not be checked.

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. MD and MF were hard to use, so 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. The early use of MX was to identify gateways to other networks, but it didn't come into wide use until the DNS was well established in the early 1990s.

RFC 5321 sec. 5 now clearly states that:

  1. SMTP clients must look up for an MX record;
  2. if no MX record for domain is present, look up for an A Resource Record (RR), and if such record is present, treat it as an MX record;
  3. if an MX record is present, clients MUST NOT use an A RR.
RFC 5321, sec. 5.1, page 69 5.1 Locating the Target Host Once an SMTP client lexically identifies a domain to which mail will be delivered for processing (as described in Sections 2.3.5 and 3.6), a DNS lookup MUST be performed to resolve the domain name (RFC 1035 ). The names are expected to be fully qualified domain names (FQDNs): mechanisms for inferring FQDNs from partial names or local aliases are outside of this specification. Due to a history of problems, SMTP servers used for initial submission of messages SHOULD NOT make such inferences (Message Submission Servers have somewhat more flexibility) and intermediate (relay) SMTP servers MUST NOT make them.

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