Kerberos Authentication for Outlook Clients
In his article, Hendrik Walter explains how to enable Kerberos Authentication for Outlook Client connecting to Exchange Server 2010 SP1 and later.
It is not so difficult as you would expect, therefore take your time and read his value article.
http://www.msexchange.org/articles_tutorials/exchange-server-2010/management-administration/enabling-kerberos-authentication-mapi-clients-connecting-exchange-2010-sp1.html