mail @example.com without dns a-record for example.com

Followup-To: comp.mail.misc

The objective here is just learning how things work.  I asked
myself---is it possible to get mail sent to someone@example.com without
an a-record fore example.com?  I think the answer should be ``yes''
because I thought an SMTP would do a type-mx dns query, learn that
example.com mail is handled by mx.example.com, would get the ip address
of mx.example.com and reach the server just fine.

So I made an experiment using my domain---example.com, say.  I've been
geting mail just fine every day.  Then I deleted the a-record for
example.com.  My mail system is not really dependent on it in any way as
far as I know.  So the experiment has the set up:

# host -t a example.com
example.com has no A record

# host -t mx example.com
example.com mail is handled by 10 mx.example.com.

# host -t a mx.example.com
mx.example.com has address 1.2.3.4

But after the deletion of the a-record example.com, I noticed Gmail
seems not to deliver emails to me anymore.  As soon as I created it back
and send a new message, it arrived just fine.

Is is just Gmail or is there more I don't understand?