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When a DNS lookup for MX records fails to complete, why doesn Exim send the messsage to the host defined by the A record?

April 26, 2017COMPLETE DNS Exim fails host lookup messsage MX records send

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When a DNS lookup for MX records fails to complete, why doesn Exim send the messsage to the host defined by the A record?

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A0313: The RFCs are quite clear on this. Only if it is known that there are no MX records is an MTA allowed to make use of the A record. When an MX lookup fails to complete, Exim does not know whether there are any MX records or not. There seem to be some nameservers (or some configurations of some nameservers) that give a “server fail” error when asked for a non-existent MX record. Exim uses standard resolver calls, which unfortunately do not distinguish between this case and a timeout, so all Exim can do is try again later.