SPF is not the problem here, SPF determines what mail server can send yahoo mail. The issue here is DMARC which determines (amongst other things) what to do whe

SPF is not the problem here, SPF determines what mail server can send yahoo mail. The issue here is DMARC which determines (amongst other things) what to do when someone puts yahoo.com in the "From:" field. Yahoo has set it up so that mail gets rejected in that case, even if you add the proper "Resent-From" header to be something non-yahoo.