+-- | Check whether the given domain has a valid MX record. NULLMX
+-- (RFC7505) records consisting of a single period must not be
+-- accepted.
+--
+-- Two points about NULLMX:
+--
+-- * RFC7505 states that a domain MUST NOT have any other MX records
+-- if it has a NULLMX record. We don't enforce this. If you have a
+-- NULLMX record and some other MX record, we will reluctantly
+-- consider the second one valid.
+--
+-- * RFC7505 also states that a NULLMX record must have a priority
+-- of 0. We do not enforce this either. We ignore any records
+-- containing an empty label (i.e. a single dot). Such a record will
+-- not be deliverable anyway, and in light of the first item, means
+-- that we will not \"incorrectly\" reject batshit-crazy domains
+-- that have a NULLMX record (but with a non-zero priority) in
+-- addition to other, valid MX records.
+--
+