How do I disable the Outlook auto-complete and clear the cache

If you are experiencing a lot of failed sent mail due to malformed addresses in your auto-complete or address book follow these instructions on how to clear your list and cache.

In Outlook 2010 and 2013, click the File menu and select Options.

In the Outlook Options window click the Mail tab.

Scroll down to Send messages. Uncheck the Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines box.

Optional Step: To clear out the auto-complete list, simply click the Empty Auto-Complete List button.

You’ll see a confirmation window about clearing the auto-complete list, click Yes.

Outlook auto-complete should be completely disabled and any history that was in the auto-complete list should be completely erased (if you clicked the clear auto-complete button).
More information about fixing nickname and autocomplete lists can be found on Microsoft's support site.
Source: http://accc.uic.edu/answer/how-do-i-disable-outlook-auto-complete-and-clear-cache