[ih] MX and WKS records

[John Levine]

Over on another list we're having yet another rerun of the argument about what
to do when you want to send mail to a domain and it has no MX records or it has
dodgy MX records and when you fall back to A or AAAA records. RFC 974 answered
these questions thirty-nine years ago but a few mysteries remain. I figure that
since the author of 974 is on this list, it's a good a place to ask as any.

If there's no MX records, you pretend there's an MX pointing at the domain
itself and use the A record. Was that intended as a temporary transition until
everyone published MX, or a permanent part of the spec? (As we all know, there
is nothing so permanent as a temporary transition hack.)

It also says to do a WKS lookup to see what services a host provides, and
in particular whether it does SMTP.  I started running mail servers in the
early 1990s and I don't ever remember publishing or looking for WKS.  Did
anyone ever use them?  

Fallback to A turned out to have the unfortunate side effect that mail to
domains with no mail service hammer on the host(s) at the A record until they
give up, typically days later. In 2015 we finally published RFC 7505, the Null
MX band-aid, the reverse of WKS saying a host does not accept mail as opposed
to WKS which said that it did.

R's,
John