Using Ubuntu Linux, I somehow have to backup my files associated to Kontact applications such as Kmail, Kalendar, and Knote. The followings are files and directories under a user home directory, used by each of the applications.
1. Kmail:
.kde/share/apps/kmail/
.kde/share/config/kmailrc
2. You can export a current Calendar in Kontact as "iCalendar" format, save it with a unique name in a directory, and add it to Calendar again. Do not need to backup the files under .kde directory. "To-do List" and "Journal" are also included in the "iCalendar" file of the ".ics" extension.
3. Knote:
.kde/share/apps/knote/