There can be a number of different causes for Outlook not to retain the list of Autocomplete addresses after it closes but I used the steps below to resolve the issue
- Close Outlook
- Press the Windows Key + R Key to open the Run command
- Enter the following address and press Ok
%LOCALAPPDATA%\Microsoft\Outlook
- Windows Explorer will now open the Outlook folder
- Right click on the RoamCache folder and select delete
- Open Outlook again and it will rebuild the RoamCache folder and autocomplete will now save email addresses