Nodoid:
That would be the case in the usual way, but if the writer had clearly indicated that the material was *not* for publication then the *publisher* becomes 100% responsible for any and all consequense, including any damaging consequenses to the writer, and cannot pass any blame back to the writer.
Copyright with letters (and hence email) is very complicated and only people with very good lawyers and lots of money to pay them should step outside the usual conventions.