This sometimes happens if you upgrade from an older version of KDE (like Slackware's 4.5.5) and keep your existing KDE configuration below "~/.kde" intact.
Can you please try the following:
(1) Launch Akonadi Console (for instance by pressing "Alt-F2" to open krunner and typing "akonadiconsole")
(2) In the "Agents" tab, select the "Local Folders" resource
(3) Select "Configure > Configure natively...".
(4) If an error appears indicating that "the current folder does not exist" don't worry. Select a new
directory which does not yet exist, for instance: /home/<USERNAME>/.kde/share/apps/kmail/mail/
This should fix your issue. The bug is fairly old, and is discussed at https://bugs.kde.org/show_bug.cgi?id=259355