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To reset the whole Outlook nickname cache for Outlook 2010 and later versions, use one of the following methods: 

Method 1

1. Open Outlook.
2. On the File tab, click Options.
   

   
   

3. Click the Mail tab.
   

   

4. Under Send Messages, click Empty Auto-Complete List.
5. Click Yes.

Outlook will generate a new nickname cache.

Method 2

1. Click Start, and then click Run.
2. Start Outlook by using the /CleanAutoCompleteCache switch. To do this, type Outlook.exe /CleanAutoCompleteCache
   
   Note If Outlook is not installed in the default location, you must point to the path of Outlook.exe.

Outlook will generate a new nickname cache.