/usr/include/KF5/mailcommon/ is in libkf5mailcommon-dev 4:16.04.2-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/KF5/mailcommon/accountconfigorderdialog.h
/usr/include/KF5/mailcommon/addtagdialog.h
/usr/include/KF5/mailcommon/backupjob.h
/usr/include/KF5/mailcommon/collectionannotationsattribute.h
/usr/include/KF5/mailcommon/collectionexpirypage.h
/usr/include/KF5/mailcommon/collectiongeneralpage.h
/usr/include/KF5/mailcommon/collectiontypeutil.h
/usr/include/KF5/mailcommon/dbusoperators.h
/usr/include/KF5/mailcommon/expirecollectionattribute.h
/usr/include/KF5/mailcommon/favoritecollectionwidget.h
/usr/include/KF5/mailcommon/filteraction.h
/usr/include/KF5/mailcommon/filteractiondict.h
/usr/include/KF5/mailcommon/filterimporterabstract.h
/usr/include/KF5/mailcommon/filterimporterbalsa.h
/usr/include/KF5/mailcommon/filterimporterclawsmail.h
/usr/include/KF5/mailcommon/filterimporterexporter.h
/usr/include/KF5/mailcommon/filterlog.h
/usr/include/KF5/mailcommon/filtermanager.h
/usr/include/KF5/mailcommon/foldercollection.h
/usr/include/KF5/mailcommon/foldercollectionmonitor.h
/usr/include/KF5/mailcommon/folderjob.h
/usr/include/KF5/mailcommon/folderrequester.h
/usr/include/KF5/mailcommon/folderselectiondialog.h
/usr/include/KF5/mailcommon/foldertreeview.h
/usr/include/KF5/mailcommon/foldertreewidget.h
/usr/include/KF5/mailcommon/foldertreewidgetproxymodel.h
/usr/include/KF5/mailcommon/incidencesforwidget.h
/usr/include/KF5/mailcommon/itemcontext.h
/usr/include/KF5/mailcommon/jobscheduler.h
/usr/include/KF5/mailcommon/kmfilterdialog.h
/usr/include/KF5/mailcommon/mailcommon_export.h
/usr/include/KF5/mailcommon/mailcommonsettings_base.h
/usr/include/KF5/mailcommon/mailfilter.h
/usr/include/KF5/mailcommon/mailinterfaces.h
/usr/include/KF5/mailcommon/mailkernel.h
/usr/include/KF5/mailcommon/mailutil.h
/usr/include/KF5/mailcommon/mdnstateattribute.h
/usr/include/KF5/mailcommon/metatype.h
/usr/include/KF5/mailcommon/pop3settings.h
/usr/include/KF5/mailcommon/redirectdialog.h
/usr/include/KF5/mailcommon/resourcereadconfigfile.h
/usr/include/KF5/mailcommon/searchpattern.h
/usr/include/KF5/mailcommon/searchpatternedit.h
/usr/include/KF5/mailcommon/searchrule.h
/usr/include/KF5/mailcommon/searchrulestatus.h
/usr/include/KF5/mailcommon/sendmdnhandler.h
/usr/include/KF5/mailcommon/snippetsmanager.h
/usr/include/KF5/mailcommon/tag.h
/usr/include/KF5/mailcommon/tagwidget.h