/usr/lib/x86_64-linux-gnu/libkmailprivate.so.5 is in kmail 4:16.04.3-4~deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libkmailprivate.so.5.2.3
/usr/lib/x86_64-linux-gnu/libkmailprivate.so.5.2.3