Microsoft Outlook maintains a nickname list that is used by both the automatic name checking feature and the automatic completion feature. The nickname list is generated automatically as users use Outlook. If the nickname cache is corrupted, Outlook may be unable to identify recipients, may offer incorrect recipients when automatically completing the email address, or may send the message to the wrong person.
Please note that the AutoComplete list for Outlook is specific to Outlook and is not shared by Outlook Web App (OWA). OWA maintains its own AutoComplete list.
Unlike earlier versions of Outlook that store the nickname cache in a file on the local computer, Outlook 2010 and later versions store the nickname cache in the user’s primary message store. This means that deleting the entire nickname cache is no longer a matter of deleting the famous .NK2 file.
To reset the whole Outlook nickname cache for Outlook 2010 and later versions, first open Outlook, go to the File tab, click Options, click the Mail tab and, under Send messages, click Empty Auto-Complete List:
Outlook will then generate a new nickname cache.
Alternatively, you can close Outlook, click the Start menu, click Run (depending on which version of Windows you are using), and start Outlook by using the /CleanAutoCompleteCache switch: Outlook.exe /CleanAutoCompleteCache
Outlook will start up and generate a new nickname cache.