I don't know much about mail servers, but would it be possible to validate the signature somehow before delivering the message? Is this impossible or impractical for some reason?
This is a bypass game on something like proof point url protection that rewrites URLs to go through a central redirect so that reputation check can be separated from reputation delivery, usage detected, and one liner URLs still work. I’m over a decade out of this space so take with a grain of salt.
The tool could do a better job inspecting “oh that’s a pgp formatted massage”. You could probably just route them all to spam and almost no one would notice.
As already mentioned, the email might also be encrypted (the most common case). Then the signature would not be available as it would be hidden under the encryption. These sorts of filtering issues are why corporate cybersecurity people, somewhat ironically, tend to dislike encrypted email. There is an opportunity here to make better email clients that treat encrypted, but unsigned, emails with suspicion.