Outlook maintains a list that is used by both the automatic name checking feature and the auto completion feature. The nickname list is generated automatically as you 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 enter their email address only instead of the name as it is saved in your Contacts.
There are several ways to resolve this, depending on the version of Office.
If you are using Outlook 2010, 2007, or 2003 click here. This will take you to Microsoft's website to download a "Fix It" Tool that will repair the issue for you.
If you are are using Outlook 2013 open up Outlook and follow these steps:
1. In the Outlook main window, click the File menu, then click the Options button.
2. In the Options window, click the Mail tab and browse to the Send Messages section. Click the Empty Auto-Complete List button.
3. A confirmation box will appear, click Yes. That's it!