/usr/share/doc/libmailutils-dev/THANKS 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/share/doc/libmailutils-dev/../libmailutils4/THANKS
/usr/share/doc/libmailutils-dev/../libmailutils4/THANKS