use the corres- pondence against the United States, not against Epstein. Rule 410^create(s), in effect, a privilege of the defendant," United States I. Mezzanatto, 513 U.S. 196, 205, 115 S.Ct. 797, 803, 130 L.Ed.2d 697 (1995) (internal quotation marks and al- teration omitted), but not a privilege of non-discl