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5.2.  "include"

      include          = "include"  ":" domain-spec

   The "include" mechanism triggers a recursive evaluation of
   check_host().  The domain-spec is expanded as per Section 8.  Then
   check_host() is evaluated with the resulting string as the <domain>.
   The <ip> and <sender> arguments remain the same as in the current
   evaluation of check_host().

   In hindsight, the name "include" was poorly chosen.  Only the
   evaluated result of the referenced SPF record is used, rather than
   acting as if the referenced SPF record was literally included in the
   first.  For example, evaluating a "-all" directive in the referenced
   record does not terminate the overall processing and does not
   necessarily result in an overall "Fail".  (Better names for this
   mechanism would have been "if-pass", "on-pass", etc.)

   The "include" mechanism makes it possible for one domain to designate
   multiple administratively-independent domains.  For example, a vanity
   domain "example.net" might send mail using the servers of