Nigel Metheringham edited this page Nov 30, 2012 · 3 revisions

Q0311

Question

When a DNS lookup for MX records fails to complete, why doesn't Exim send the messsage to the host defined by the A record?

Answer

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 name servers (or some configurations of some name servers) 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.


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