/usr/lib/x86_64-linux-gnu/libmu_auth.so is in libmailutils-dev 1:2.99.99-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmu_auth.so.4.0.0
/usr/lib/x86_64-linux-gnu/libmu_auth.so.4.0.0