June 2018
Intermediate to advanced
368 pages
11h 1m
English
Content preview from Managing Mission - Critical Domains and DNS
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Special case MX records
If there is no MX record present, the protocol is to try to deliver the email to the A or AAAA record in the destination. For example, an email is sent to markjr@antisocial.dom but antisocial.dom has no MX records. The MTA will then do an A-record lookup for antisocial.dom and attempt delivery there.
There is also the special case of the null MX (formalized in RFC7505) which is defined as a ".", preference 0 and that means no email is expected nor accepted for this domain. MTAs are supposed to hard fail and return a Non-Delivery Notifcation (NDN) when encountering email addressed to a domain with the null MX set.
You would probably define that in conjunction with the SPF record in the domain that disavows all email ...
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