I think "inside the envelope" is still too obscure. Maybe better (but getting strained) to say the reporter had a secretary who usually opens his mail and only

I think "inside the envelope" is still too obscure. Maybe better (but getting strained) to say the reporter had a secretary who usually opens his mail and only sends him a summary of his correspondence based on keywords he's interested in. But when he opens his own mail one day, he sees it has confidential information included in it, which his secretary has not been forwarding because it doesn't match the keywords. Overall, I agree with the person who said analogies make it more confusing.