/usr/lib/x86_64-linux-gnu/libkmailprivate.so.5 is in kmail 4:15.12.3-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libkmailprivate.so.5.1.3
/usr/lib/x86_64-linux-gnu/libkmailprivate.so.5.1.3