After upgrading to Windows 10 Autocomplete is not working in Outlook 2003.
Note: Office 2003 is not supported in Windows 10. It is highly recommend you upgrade to a newer version.
There is a fix to this problem. But I still highly recommend you upgrade your office suite to the latest version!
First of all open up Outlook 2003. Go to Tools > Options. Now select the Mail format tab.
Here you need to uncheck “Use Microsoft Office Word 2003 to edit e-mail messages”