+--
+-- NULLMX (RFC7505) records consisting of a single period must not
+-- be accepted. Moreover, the existence of a NULLMX must be reported
+-- back to the caller because the whole point of a NULLMX is that
+-- its existence should preempt an @A@ record check. We abuse the
+-- return type for this, and return @Nothing@ in the event of a
+-- NULLMX. Otherwise we return @Just True@ or @Just False@ to
+-- indicate the existence (or not) of MX records.
+--
+-- RFC7505 states that a domain MUST NOT have any other MX records
+-- if it has a NULLMX record. We enforce this. If you have a NULLMX
+-- record and some other MX record, we consider the set invalid.
+--
+validate_mx :: Resolver -> Domain -> IO (Maybe Bool)