A null record in your SPF record is commonly an indication of a problem with the related DNS lookup. In general, any mechanism that contains a DNS lookup should return a result. We trigger this sub-action because it's much more common for a null record to be a problem affecting your email delivery over there being a null record on purpose like those in a Void Lookup
You can generally run an A record lookup on the domain/sub-domain in question to get more specific results.
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