Doubts about DKIM verification (RFC6376)
Solution 1:
You have it yourself:
Survivability of signatures after transit is not guaranteed, and signatures can fail to verify through no fault of the Signer.
I read it this way: A sends to C. The message is relayed via B so the flow looks like A->B->C
Party B can take steps (such as stripping off or changing headers) that invalidates the signature. Penalizing party A attempting to the right thing for B's bad behavior makes no sense, and so should be treated no differently then a message without a signature at all.