[Gagarin1917]

Correspondence between a person and a business should be considered “papers.”

[twoodfin]

That’s not how it was understood at the time of ratification. Letters and the post office were a thing then, mentioned elsewhere in the Constitution. If the framers of the 4th amendment wanted it to cover them they would have written the amendment differently.

[Gagarin1917]

But sealed letters ARE protected under the 4th Amendment. A warrant is necessary to open all sealed mail.