When Outlook 2016 client connects to Exchange Server by using Autodiscover service with Exchange ActiveSync (EAS), it does not use the MAPI/HTTP protocol that sends an anonymous HTTP request, as expected. Instead, it initially tries to do BASIC authentication by using an account without a password.
To fix this issue, install the March 6, 2018, update for Outlook 2016 (KB4018296), and then set the following registry key to disable the EAS Autodiscover feature in Outlook 2016 client.
Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
- Exit all Microsoft Office applications.
- Start Registry Editor:
- In Windows 10, go to Start, enter regedit in the Search Windows box, and then select regedit.exe in the search results.
- In Windows 7, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
- In Windows 8 and Windows 8.1, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
- Locate and then select the following registry subkey: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Setup\
- On the Edit menu, point to New, and then click DWORD Value.
- Type EnableEASAccountCreation, and then press Enter.
- In the Details pane, right-click EnableEASAccountCreation, and then click Modify.
In the Value data box, type 0, and then click OK.
- Exit Registry Editor.