>Since yesterday, when typing in the To field, the auto complete does not work as before. It used to show me, in a different interface, the To emails from my address book. Now, it shows the entire list of To which I ever sent emails to, in a not so interesting dropdown list. Anyone knows anything about it and if this is related to a setting that I could have mistakenly changed?

Some comments in no particular order:

1. For me, Outlook only shows address book(s) "in a different interface" if I click on the "To" button to the left of the field itself. If I start typing an address directly in the To field, I get a "Recent People" dropdown list below the field itself, which gets filtered as I type

2. If I do click the "To" button, I can select the address book I'd like to search at the top right. If you're doing that you may, by default, be looking at a different address book than usual

3. Microsoft Office did install an update for me yesterday. The behaviour above is what I'm seeing today, I don't see any difference from what I had earlier

4. If you've recently created a new user profile on your computer, or a new Outlook profile, by default you will lose your previous autocomplete list/cache unless you take steps to preserve and restore it: https://docs.microsoft.com/en-us/outlook/troubleshoot/contacts/information-about-the-outlook-autocomplete-list . If you don't do this, autocomplete will eventually re-learn and become useful again