I've been using KMail for a month or two now and have so far had no problems with it. However, today my laptop died after the battery ran out of power, and since then whenever I log in I see a steady 100% CPU use from the following processes:

akonadi_mailfilter_agent
akonadi_imap_resource
akonadi_nepomuk_feeder

Running "akonadictl stop" kills the processes and returns CPU usage to reasonable levels, but then KMail complains that it cannot run without akonadi. Can I clear some type of cache or other settings to get KMail back on its feet?

After some additional hacking around, I found a solution:

Run akonadictl stop to stop akonadi
Remove everything under ~/.local/share/akonadi
Remove everything under ~/.config/akonadi
Restart akonadi

After restarting akonadi, it took a good 15 minutes to resync everything. However, the CPU usage during the sync was about 40% per daemon, much better than 100%.