I have made a work-around to fix this problem. From my laptop, before converting to Win 10, I exported my Yahoo contact list and saved the file to a thumb drive. Then I went to my desktop where I have already upgraded to Win 10 and created an account in Outlook. Next I went into Outlook and imported the Yahoo contact (CSV format) file from my thumb drive and added the Outlook account to my Mail app. This seems to work fine when I send mail from Yahoo, I now get suggested addresses from the full contact file that I uploaded. What I don't know yet is if the contact list will be updated when I send new addressees email from my Yahoo account.