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

Fri Apr 4 19:40:55 UTC 2003

On Thu, Apr 03, 2003 at 05:25:35PM -0500, Richard Irving wrote:
>   It isn't exactly completely RFC compliant, but, it is only a -=Request=-, eh ?

It is in fact required that an MTA fall back to the A record for a domain if
an MX record does not exist.  See RFC 2821, Section 5, "Address Resolution 
and Mail Handling".

> > Obviously some admins I have encountered are starting to host mailservers
> > for sub-domains and domains without MX entries on their DNS zone records.
> > Relying on the A record alone.
> 
>    Lemmings make a mad dash towards a cliff, every so often, en masse....

This is a fallacy perpetrated by Disney.

http://www.snopes.com/disney/films/lemmings.htm

--Adam