"Private correspondence" refers to written or electronic communication between individuals that is meant to be kept confidential and not shared with others. Full definition
The most valuable contributions so far have been set down only in private correspondence, and have not as yet been published. (religion-online.org)
We translate private correspondence, web - sites, different research works! (russianwomeninfo.com)
All I see is a second hand report of private correspondence between Dr. Wegman and Dr. Rapp, and nowhere in there do I see a question asked directly to Dr. Bradley. (deepclimate.org)