If you are experiencing a lot of failed sent mail due to malformed addresses in your AutoComplete or address book follow these instructions on how to clear your list and cache.
To disable AutoComplete or clear your AutoComplete list entirely, follow these steps:
- In Outlook 2010/2013/2016, 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.
To clear the AutoComplete list, simply click the Empty AutoComplete List button. You’ll see a confirmation window about clearing the AutoComplete list, click Yes.
Outlook AutoComplete 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).