/usr/include/KF5/mailimporter/ is in libkf5mailimporter-dev 4:17.12.3-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/KF5/mailimporter/filterbalsa.h
/usr/include/KF5/mailimporter/filterclawsmail.h
/usr/include/KF5/mailimporter/filterevolution.h
/usr/include/KF5/mailimporter/filterevolution_v2.h
/usr/include/KF5/mailimporter/filterevolution_v3.h
/usr/include/KF5/mailimporter/filtericedove.h
/usr/include/KF5/mailimporter/filterimporterbase.h
/usr/include/KF5/mailimporter/filterinfo.h
/usr/include/KF5/mailimporter/filterinfogui.h
/usr/include/KF5/mailimporter/filterkmail_maildir.h
/usr/include/KF5/mailimporter/filterkmailarchive.h
/usr/include/KF5/mailimporter/filterlnotes.h
/usr/include/KF5/mailimporter/filtermailapp.h
/usr/include/KF5/mailimporter/filtermailmangzip.h
/usr/include/KF5/mailimporter/filtermbox.h
/usr/include/KF5/mailimporter/filteroe.h
/usr/include/KF5/mailimporter/filteropera.h
/usr/include/KF5/mailimporter/filterplain.h
/usr/include/KF5/mailimporter/filterpmail.h
/usr/include/KF5/mailimporter/filters.h
/usr/include/KF5/mailimporter/filterseamonkey.h
/usr/include/KF5/mailimporter/filtersylpheed.h
/usr/include/KF5/mailimporter/filterthebat.h
/usr/include/KF5/mailimporter/filterthunderbird.h
/usr/include/KF5/mailimporter/importmailswidget.h
/usr/include/KF5/mailimporter/mailimporter_export.h
/usr/include/KF5/mailimporter/mailimporter_utils.h
/usr/include/KF5/mailimporter/messagestatus.h
/usr/include/KF5/mailimporter/othermailerutil.h
/usr/include/KF5/mailimporterakonadi/filterimporterakonadi.h
/usr/include/KF5/mailimporterakonadi/mailimporterakonadi_export.h