sendmail, 4th Edition

Ambiguous MX Records

RFC974 leaves the treatment of ambiguous MX records to the implementor’s discretion. 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. Such a configuration results in hostB never looking up any MX records because it recognizes mail to foo as local.

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. Clearly, hostB should have been configured to recognize foo as local. It didn’t (hence the MX lookup and discarding in the first place), ...