An A record is an MX record and is a missing MX....

Of all the gin joints in all the towns in all the world, Richard Irving
had to walk into mine and say:

   Agreed, but nothing -requires- an MTA Agent have an MX record, in the first
  place.... it is just a best CBP. Not having one means you don't comply
  with ALL the RFC, but you are still RFC compliant. Not the same thing, FWIW.

As part of continuing escalations in the war on spam, some MTAs are now
refusing to *accept* mail when the sender's domain does not have MX
records defined.

Whether this is a *wise* decision is becoming unimportant; people are
doing it.