This has generated much debate in sendmail circles. Consider the following:
foo IN MX 10 hostA
foo IN MX 20 hostB ?†? mail from hostB to foo
foo IN MX 30 hostC
When mail is sent from a host (hostB) that is an MX record for the receiving host
(foo) all MX records that have a cost equal to or greater than that of hostB must be
discarded. The mail is then delivered to the remaining MX host with the lowest cost
(hostA). This is a sensible rule because it prevents hostB from wrongly trying to
deliver to itself.
It is possible to configure hostB so that it views the name foo as a synonym for its
own name. Sucha configuration results in hostB never looking up any MX records
because it recognizes mail to foo as local.
This is the Title of the Book, eMatter Edition
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338 | Chapter 9: DNS and sendmail
But what should happen if hostB does not recognize foo as local and if there is no
hostA?
?†? no hostA
foo IN MX 20 hostB ?†? mail from hostB to foo
foo IN MX 30 hostC
Again, RFC974 says that when mail is being sent from a host (hostB) that is an MX
record for the receiving host (foo) all MX records that have a cost equal to or greater
than that of hostB must be discarded. In this example, that leaves zero MX records.
Three courses of action are now open to sendmail, but RFC974 doesn??™t say which it
should use:
??? Assume that this is an error condition.
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