Clear Outlook Cache

To delete a single cached entry:

In Outlook:

1. Open Outlook.
• Outlook 2010/2013: On the Home page, click New Email.
• Outlook 2002/2003/2007: Click New > Mail Message.
2. Type one or more letters of the recipient name or address; this will show memorized (cached) entries in a drop-down list.

Use the arrow keys on your keyboard to select the entry to be deleted. With the entry highlighted, press the DELETE key on your keyboard.
This removes the entry from your autocompletion cache.

To delete all cached entries:

In Outlook 2010/2013:

1. Navigate to File > Options > Mail.

Scroll down to Send Messages and click Empty Auto-Complete list:

Alternatively, you can download Microsoft 'Fix It' tool which will clear the nickname cache automatically for all versions of Outlook.