There seems to be a little confusion in questions I have gotten about
the "Try Parent Domain" step and the title of this straw poll. In
particular, whether the parent domain check is intended to apply to
subdomains at all.
The confusion is whether a hostname, like www.example.com, is a
subdomain or not:
(1) When used in an email address, www.example.com is called a domain,
and since it's below example.com, it could be called a subdomain of
example.com.
(2) In some other contexts, a hostname may not be considered to be a
subdomain, since it has nothing below it. If there's a
www.eng.example.com, then eng.example.com would be a subdomain
(regardless of whether it has its own SOA record, or a direct record for
www.eng exists within the example.com zone).
Using definition (2), the parent domain check is beneficial only for
hostnames, but not for subdomains. Since any hostname can receive mail
via its A or AAAA record, in order to achieve complete coverage for ADSP
within a given domain, every hostname needs its own ADSP record, absent
a reference to the ADSP record at the next higher level. The parent
domain check does not attempt to cover subdomains, but since they are
much less numerous than hostnames, the overhead of publishing ADSP
records (manually, if necessary) for subdomains is much less than for
hostnames.
-Jim
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