/usr/include/mailtransport/ is in kdepimlibs5-dev 4:4.13.0-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/mailtransport/dispatcherinterface.h
/usr/include/mailtransport/dispatchmodeattribute.h
/usr/include/mailtransport/errorattribute.h
/usr/include/mailtransport/mailtransport_export.h
/usr/include/mailtransport/messagequeuejob.h
/usr/include/mailtransport/precommandjob.h
/usr/include/mailtransport/sendmailjob.h
/usr/include/mailtransport/sentactionattribute.h
/usr/include/mailtransport/sentbehaviourattribute.h
/usr/include/mailtransport/servertest.h
/usr/include/mailtransport/smtpjob.h
/usr/include/mailtransport/socket.h
/usr/include/mailtransport/transport.h
/usr/include/mailtransport/transportattribute.h
/usr/include/mailtransport/transportbase.h
/usr/include/mailtransport/transportcombobox.h
/usr/include/mailtransport/transportconfigdialog.h
/usr/include/mailtransport/transportjob.h
/usr/include/mailtransport/transportmanagementwidget.h
/usr/include/mailtransport/transportmanager.h
/usr/include/mailtransport/transporttype.h