This seems interesting enough for a first post.
I had previously spent all afternoon trying to figure out how to restore a customer's email address auto-comlpete function to a new computer. He had subsequently sent the computer back to HP, and the nice folks at HP had reinstalled the OS even though the problem was that the speakers jack had failed.
HP had also installed Microsoft Office for him, though obviously not the same one version. His previously working connection to his contacts on iCloud no longer worked. It took a while to figure out with the help of a slipstick.com article (https://www.slipstick.com/outlook/icloud-error-outlook-configured-default-profile/) that the version HP had installed was not compatible. Downloading the "correct" version using his Microsoft account didn't help, but the same slipstick.com article had the steps to fix the problem.
I had spent way too much time on the iCloud problem, so I did a quick check of the auto-complete, and it seemed to be working. I left thinking all was well.
I got an email from the customer a day later. Apparently, everything he expected to be in auto-complete was not there. The customer had spent the afternoon watching me do Google searches to find answers to the many questions I could not answer myself. He took it upon himself to try to get an answer on how to fix auto-complete. He found this article: https://www.msoutlook.info/question/filling-the-autocomplete-list-with-contact-addresses.
The procedure involves sending a message to all of your contacts - embarrassing that I didn't think of it. Of course, it assumes that the user is sufficiently diligent to keep his contacts up to date.