How to turn on or turn off AutoComplete

Outlook 365
The AutoComplete feature generates an AutoComplete list, also known as the nickname cache. This list is generated automatically after sending email messages from Outlook:
New message in Outlook 2016

For a specific situation, you can turn this feature off, temporarily or permanently. To turn on or turn off the AutoComplete feature, do the following:

   1.   On the File tab, click the Options button:

Outlook 2016 Options

   2.   In the Outlook Options dialog box, on the Mail tab, under Send messages, check or uncheck the Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines option to turn AutoComplete on or off:

Send messages Options in Outlook 2016

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See Also

Improved AutoComplete

Improved AutoComplete

Outlook uses two features to resolve and complete names entered into email messages: AutoComplete and AutoResolution.
Add entries to the AutoComplete list

Add entries to the AutoComplete list

The AutoComplete list is updated automatically after sending an email (see more about AutoComplete feature). To make it easier to work in Outlook, you can add useful email addresses to your AutoComplete list without sending emails.
Remove entries from the AutoComplete list

Remove entries from the AutoComplete list

By default, Outlook uses the AutoComplete list to suggest email addresses. If the AutoComplete feature is not disabled in Outlook (see how to turn it off), the AutoComplete list is updated automatically after sending an email. From time to time, people make mistakes when entering addresses or receive incorrect addresses for use.