The Auto-Complete list, also known as the nickname cache, is generated automatically when you send email messages from Outlook. The list contains email addresses and display names for people to whom you have sent mail previously.
If one or more recipient addresses are incorrect, it may be necessary to clear the Auto-complete list.
To clear the list, follow the steps below:
In the Outlook top menu bar, click File 
Click Options
- From the left hand menu, click Mail
- Scroll down to the Send messages section
- Click the Empty Auto-Complete List button
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