Hello @SteveJ144 ,
Are you using the actual share links for the files? For files hosted in a cloud drive, whether that's in the Library on our side or through another service, what appears in the address bar isn't generally what you will actually be linking to. Anything hosted in cloud drives should, generally speaking, have a specific share URL, where you can also potentially add accessibility restrictions.
I'd advise checking the links you're using for the button in the email, to make sure those are the proper share links for the documents. Otherwise, you can just host the documents on our side, and link to them in your email much more directly.
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