/usr/lib/x86_64-linux-gnu/libmu_pop.so is in libmailutils-dev 1:3.4-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmu_pop.so.5.0.5
/usr/lib/x86_64-linux-gnu/libmu_pop.so.5.0.5