If you experience the problem that KMail / Kontact 4.9.x are not doing address autocompletion in the composer window, you can edit
~/.kde4/share/config/kpimcompletionorder on some systems) and add
Beware: This is only a workaround and can cause lock-ups of the composer window. Instead of using nepomuk, kontact should be using akonadi to directly and only query you address books. Using nepomuk will query all you indexed data, such as emails and documents, and depending on the amount, the performance can be problematic. That’s why nepomuk is disabled by default.