/usr/include/mailutils/cpp/ is in libmailutils-dev 1:2.2+dfsg1-5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/mailutils/cpp/address.h
/usr/include/mailutils/cpp/attribute.h
/usr/include/mailutils/cpp/body.h
/usr/include/mailutils/cpp/debug.h
/usr/include/mailutils/cpp/envelope.h
/usr/include/mailutils/cpp/error.h
/usr/include/mailutils/cpp/filter.h
/usr/include/mailutils/cpp/folder.h
/usr/include/mailutils/cpp/header.h
/usr/include/mailutils/cpp/iterator.h
/usr/include/mailutils/cpp/list.h
/usr/include/mailutils/cpp/mailbox.h
/usr/include/mailutils/cpp/mailcap.h
/usr/include/mailutils/cpp/mailer.h
/usr/include/mailutils/cpp/mailutils.h
/usr/include/mailutils/cpp/message.h
/usr/include/mailutils/cpp/mime.h
/usr/include/mailutils/cpp/mutil.h
/usr/include/mailutils/cpp/pop3.h
/usr/include/mailutils/cpp/registrar.h
/usr/include/mailutils/cpp/secret.h
/usr/include/mailutils/cpp/sieve.h
/usr/include/mailutils/cpp/stream.h
/usr/include/mailutils/cpp/url.h