/usr/lib/x86_64-linux-gnu/ is in libmailutils-dev 1:2.99.99-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/x86_64-linux-gnu/libmu_dbm.so
/usr/lib/x86_64-linux-gnu/libguile-mailutils-v-2.99.99.so
/usr/lib/x86_64-linux-gnu/libmu_py.so
/usr/lib/x86_64-linux-gnu/libmu_scm.so
/usr/lib/x86_64-linux-gnu/libmu_maildir.so
/usr/lib/x86_64-linux-gnu/libmu_mh.so
/usr/lib/x86_64-linux-gnu/libmu_cfg.so
/usr/lib/x86_64-linux-gnu/libmailutils.so
/usr/lib/x86_64-linux-gnu/libmu_sieve.so
/usr/lib/x86_64-linux-gnu/libmu_pop.so
/usr/lib/x86_64-linux-gnu/libmu_argp.so
/usr/lib/x86_64-linux-gnu/libmu_auth.so
/usr/lib/x86_64-linux-gnu/libmu_compat.so
/usr/lib/x86_64-linux-gnu/libmu_mbox.so
/usr/lib/x86_64-linux-gnu/libmu_mailer.so
/usr/lib/x86_64-linux-gnu/libmuaux.so
/usr/lib/x86_64-linux-gnu/libmu_imap.so