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Think in terms of a general SpamAssassin plug-in that takes advantage of your X-Authent header. It isn't going to know whether or not to expect the header or not (unless specifically configured).
Using X-Authent: as the border it would have to be:
a. Specially configured to know where to expect it.
b. Dependent on you to 100% strip the old header and replace it with the new one.
No, keeping in mind that not all mail an MDA might get would necessarily come through your service and this starts to get complex.
All that complexity is avoided by putting your header above Received:. Why would you possibly not want to do it that way?
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