Exchange 2010 now provides a server side cache for auto-completion of e-mail addresses. In previous versions such as exchange 2003 and exchange 2007 auto-complete data was stored locally in the outlook profile on the local PC in an NK2 file. If the users outlook profile was ever recreated, all their auto complete data was lost.
Because this auto complete data is stored on the server, it is available to the user regardless which PC they are using. Exchange 2010 also makes this auto complete data available to mobile devices running Windows Mobile Device version 6.5 or higher.
To import your NK2 files into Outlook 2010 please see: