/usr/include/mailutils/sys/ is in libmailutils-dev 1:2.99.98-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/mailutils/sys/amd.h
/usr/include/mailutils/sys/attribute.h
/usr/include/mailutils/sys/auth.h
/usr/include/mailutils/sys/body.h
/usr/include/mailutils/sys/dbm.h
/usr/include/mailutils/sys/debcat.h
/usr/include/mailutils/sys/envelope.h
/usr/include/mailutils/sys/file_stream.h
/usr/include/mailutils/sys/filter.h
/usr/include/mailutils/sys/folder.h
/usr/include/mailutils/sys/gsasl-stream.h
/usr/include/mailutils/sys/header.h
/usr/include/mailutils/sys/header_stream.h
/usr/include/mailutils/sys/imap.h
/usr/include/mailutils/sys/imapio.h
/usr/include/mailutils/sys/iostream.h
/usr/include/mailutils/sys/iterator.h
/usr/include/mailutils/sys/list.h
/usr/include/mailutils/sys/logstream.h
/usr/include/mailutils/sys/mailbox.h
/usr/include/mailutils/sys/mailer.h
/usr/include/mailutils/sys/mapfile_stream.h
/usr/include/mailutils/sys/memory_stream.h
/usr/include/mailutils/sys/message.h
/usr/include/mailutils/sys/message_stream.h
/usr/include/mailutils/sys/mime.h
/usr/include/mailutils/sys/monitor.h
/usr/include/mailutils/sys/msgset.h
/usr/include/mailutils/sys/nntp.h
/usr/include/mailutils/sys/nullstream.h
/usr/include/mailutils/sys/observer.h
/usr/include/mailutils/sys/pop3.h
/usr/include/mailutils/sys/prog_stream.h
/usr/include/mailutils/sys/property.h
/usr/include/mailutils/sys/rdcache_stream.h
/usr/include/mailutils/sys/registrar.h
/usr/include/mailutils/sys/smtp.h
/usr/include/mailutils/sys/stream.h
/usr/include/mailutils/sys/streamref.h
/usr/include/mailutils/sys/streamtrans.h
/usr/include/mailutils/sys/syslogstream.h
/usr/include/mailutils/sys/temp_file_stream.h
/usr/include/mailutils/sys/tls-stream.h
/usr/include/mailutils/sys/url.h
/usr/include/mailutils/sys/xscript-stream.h