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That is not exactly how SPF was meant to use. If it fails email should be rejected not scored. Disclaimer: I do the same as you.

So the developer of SPF (see jedberg above) says it should be used as a ranking factor, but SPF itself says it should be rejected?

Maybe the issue with SPF is in the messaging around it?

I think that would depend on whether the SPF record indicated soft-fail or hard-fail, no?