/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
/usr/include/KF5/MailImporter/FilterClawsMail
/usr/include/KF5/MailImporter/FilterEvolution
/usr/include/KF5/MailImporter/FilterEvolution_v2
/usr/include/KF5/MailImporter/FilterEvolution_v3
/usr/include/KF5/MailImporter/FilterIcedove
/usr/include/KF5/MailImporter/FilterImporterBase
/usr/include/KF5/MailImporter/FilterInfo
/usr/include/KF5/MailImporter/FilterInfoGui
/usr/include/KF5/MailImporter/FilterKMailArchive
/usr/include/KF5/MailImporter/FilterKMail_maildir
/usr/include/KF5/MailImporter/FilterLNotes
/usr/include/KF5/MailImporter/FilterMBox
/usr/include/KF5/MailImporter/FilterMailApp
/usr/include/KF5/MailImporter/FilterMailmanGzip
/usr/include/KF5/MailImporter/FilterOE
/usr/include/KF5/MailImporter/FilterOpera
/usr/include/KF5/MailImporter/FilterPMail
/usr/include/KF5/MailImporter/FilterPlain
/usr/include/KF5/MailImporter/FilterSeaMonkey
/usr/include/KF5/MailImporter/FilterSylpheed
/usr/include/KF5/MailImporter/FilterTheBat
/usr/include/KF5/MailImporter/FilterThunderbird
/usr/include/KF5/MailImporter/Filters
/usr/include/KF5/MailImporter/ImportMailsWidget
/usr/include/KF5/MailImporter/MailImporter_Utils
/usr/include/KF5/MailImporter/MessageStatus
/usr/include/KF5/MailImporter/OtherMailerUtil
/usr/include/KF5/MailImporterAkonadi/FilterImporterAkonadi