In message <19980706222427.08761@www.fundy.ca>, David Maxwell writes:
>The RFCs specifically state that an MTA MUST NOT accept a message unless
>it can guarantee being able to return error messages. By using a From: address
>with a host portion which has no MX, the mailer cannot be assured of its 
>ability to return errors.

I was under the impression that the official behavior was to use the A record
as an MX if there isn't an MX listed.

-s