You can always view your contacts in Outlook by clicking the Address Book button or using the hotkey Ctrl+Shift+B ( ⌘+0 on MacOS ). You can also see your contacts at a glance using the contacts pane (hotkey Ctrl+3 or ⌘+3 on MacOS ). It should be noted that if you’re using an Exchange type email account (Office 365 for example) that there can be multiple parts of your address book where contacts can be saved. You can see the multiple parts by opening the Address Book then clicking the drop-down menu titled Address Book.
If you’re ever in a situation where you think you may have lost your contacts, these are the locations you should check first. Generally speaking it’s rare that people lose some or all of their contacts, it’s usually the case that you don’t have a contact saved in Outlook’s auto-complete data that causes most people to think they’ve lost their contact(s).
If you ever want to turn off the auto-complete functionality in Outlook for any reason, that is also possible. You can find the option by going to File > Options > Mail and then scrolling down a bit and looking for the “Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines” option – un-check it to disable the feature.
You’ll also find the Empty Auto-Complete List button in that location which you can use to clear the data, this can be a useful troubleshooting step as well if you’re running into issues with auto-complete in Outlook.
Note: If you use Outlook for MacOS there is no setting for the Auto-Complete cache under Outlook > Preferences. The only way you can remove entries from the cache is when to click the X button beside the listed name when you’re typing in a text field. See the picture below for further clarification, for more information on the Auto-Complete cache on MacOS see this article from Microsoft.
How to delete an entry from the auto-complete cache when using MacOS