Editing MX record (section)

==Fallback to the address record==
In the absence of an MX record, email senders will attempt delivery to the address record - e.g. example.com.

This is based on RFC 5321 sec. 5.1, which states :
* SMTP clients must look up an MX record;
* If (''and only if'') no MX record for the domain is present, treat the domain as if it had an MX record with the given domain as the target hostname and a preference value of 0
* Perform A or AAAA lookups as required to determine the IP address of the target hostname

===Historical background===
RFC 821 was published in 1982. It makes only passing references to DNS, because at the time the transition from [[Hosts (file)#History|HOSTS.TXT]] to the DNS had not yet started. RFC 883, the first description of the DNS, was published 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 was not scheduled to be phased out until the end of 1985 and was not 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 [[List of DNS record types|WKS]] lookup<ref>{{cite IETF |title=MAIL ROUTING AND THE DOMAIN SYSTEM |rfc=974 |author=Craig Partridge |date=January 1986 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=18 November 2011 |quote = For each MX, a WKS query should be issued to see if the domain name listed actually supports the mail service desired.  MX RRs which list domain names which do not support the service should be discarded.  This step is optional, but strongly encouraged. }}</ref> on each MX host to see if it actually supports SMTP and discard the MX entry if not. 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 would not 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 did not come into wide use until the DNS was well established in the early 1990s.<ref>This section is adapted from [[John Levine]] [http://www.imc.org/ietf-smtp/mail-archive/msg04438.html ietf-smtp message] {{webarchive|url=https://web.archive.org/web/20080601005436/http://www.imc.org/ietf-smtp/mail-archive/msg04438.html |date=2008-06-01 }}</ref>