Outlook stops auto filling / completing email addresses

Outlook New

1. Click File, then Settings, then General and then Privacy and data.
2. If you have more than one account, you can select the account These settings apply to
3. The Suggested people section explains the feature.
4. You can backup your data by selecting Download my expanded people data
5. Select Clear data and turn off to remove all existing expanded contact data

• After 48 hours, re-enable the Activate expanded contacts suggestions option. Outlook will resume building the auto-fill list from scratch, and addresses will once again populate as suggestions after their first use.

Outlook Classic

1. Click File, then Options and then Mail.
2. Scroll down to Send messages. 
3. Make sure Use Auto-Complete List is selected. 
4. If it is off, turn it on and then click OK.

• If it is already on and your auto-fill is still not working, click Empty Auto-Complete List button and then click OK to reset your auto-fill list. You will have to type each email address the first time you send to it, but after that it will be remembered and auto-fill/complete as expected.
• This link here will also provide information about the auto complete list.