Outlook 2016 Keeps Prompting For Credentials (Username and Password)

Answer

Let’s take a look at some typical situations in which Outlook can ask the user to re-enter the password.

Saved passwords

Check if you have any passwords stored in the Windows Password Manager (Credential Manager), try to delete them all.

To directly access Credential Manager, run the command:

rundll32.exe keymgr.dll,KRShowKeyMgr

“Always Prompt for Credentials” Option in Outlook

Open Outlook Account Settings (File -> Account Settings -> Account Settings), double click on your Exchange account. Go to More settings -> select Security tab. In the User identification section clear the checkbox Always prompt for credentials.

Outlook Anywhere and NTLM authentication

If Outlook is configured to access the Exchange mailbox using Outlook Anywhere (MAPI over HTTP), verify that NTLM authentication is used for authentication. At the same time, you need to check the authentication type used for the Exchange site on the IIS.
If the computer with a problem Outlook is not joined to the domain, then on the contrary, you should try switching from NTLM authentication to Basic.

Disable Autodiscover Endpoint Check for Office 365

In addition, there is information that in one of the latest updates for Outlook 2016, mandatory checking of the connection point to the Office 365 was enabled.

You can verify this by using the utilities Fiddler or TCPView, keeping track of the attempts to connect to the servers autodiscover-s.outlook.com and outlook.office365.com.

To disable this check, go to the registry key HKEY_CURRENT_USER\Software \Microsoft\Office\16.0\Outlook \AutoDiscover and create a DWORD parameter named ExcludeExplicitO365Endpoint and value 1. Restart Outlook.

The ExcludeExplicitO365Endpoint registry parameter is applicable to Outlook 2016 version 16.0.6741.2017 and higher (this parameter is additional to the list of parameters that determine the types of checks when Outlook starts: see an article Slow Exchange AutoDiscover in Outlook 2016).

You can make changes to the registry with the following command:

reg add HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeExplicitO365Endpoint /d 1
Or, using the PowerShell cmdlet Set-ItemProperty:
Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Outlook\AutoDiscover" -Name 'ExcludeExplicitO365Endpoint' -Value 1 -Type DWORD –Force

Recreate Your Outlook Profile

If all of these methods above did not help you get rid of the problem, delete and recreate your Outlook profile.

Check Your Antivirus

Perhaps the problem is caused by your antivirus or firewall software. Try to temporarily disable it and check whether Outlook continues to prompts a password

Bad network connection

A bad network connection to the Exchange server and / or a domain controller can also be a source of re-requesting the password. Also check the computer time, it should differ from the time domain for no more than 5 minutes. If more, check the NTP time synchronization scheme in your domain.