Shangwu's comment is the one to follow here. To restore the auto complete function the simplest way is to switch Outlook to Offline mode, then open a new email form, click 'To' and add all the names you wish from your address book. Once done click send and it will be delivered to your Outbox. Delete the message from the Outbox and then return to Online Mode. Miraculously all your auto complete addresses will be back.

ada solusi yang lebih permanen kah? kalo dengan menambah server email bagaimana caranya? apakah bisa menambahkan daftar kontak secara otomatis?

this only works temporarily, when i try to restart the auto-complete doesn't work anymore

By changing Outlook to offline and making the new message to be sent (moving the message to the outbox folder) can make the auto-complete cache persistent. Of course you need to delete the message in the outbox before resuming Outlook to online.

This works temporarily, but upon Outlook restart, no longer works. I need a solution that persists across Outlook restarts. I'm using Outlook 2016.

This did not work for me. Most of my contacts do auto-complete. There is a new employee with our company that I have to type the full address each time, so it's the only one that does not auto-complete despite numerous emails already sent to this person.