What is the correct DNS setup for a host with no mail service?

If a host has no MX record, the sending MTA does a second DNS query for an A Record and then tries to connect to the IP given in the A Record response. This conversation goes something like:
Sender: give me MX records for host
Nameserver: host does not have any MX records
Sender: then, give me A Records for host
Nameserver: host's address is IP
Sender: Open a mail connection to IP
IP: ok, if set up to accept mail for host, otherwise
IP: error: IP does not accept mail for host

This behaviour is as defined in the standards and provides a solution to the situation where it was forgotten to set up MX records for hosts running mail services.

However, for a host which does not have any MX Records because it does run a mail service, this behaviour uses up unnecessary resources.

I have searched everywhere, but so far, I have been unable to find out how to set up DNS such that the conversation goes:
Sender: give me MX records for host
Nameserver: host does not accept mail

Maybe it is not implemented, or maybe it is not widely documented because it's self-explanatory to most people (but not to me!).

In any case, I would welcome any information or advice on this topic.

 

 

 

 

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