You cannot successfully set up an Office 365 Exchange Online email account in Microsoft Outlook.
This issue might occur if your Exchange administrator enables multi-factor authentication (MFA) for your account, but does not also enable modern authentication for the Exchange tenant organization.
When this issue occurs, the server returns an HTTP 456 authentication error.
To fix this issue, disable MFA for the account in the Office 365 admin center. To do this, follow these steps.
You might have to contact your Exchange administrator to disable the MFA.
- Browse to the Office 365 portal, and sign in to your Office 365 subscription by using your Global Administrator account.
- On the main portal page, select Admin.
- In the navigation pane, select Users > Active users.
- In the Active users pane, select More > Multi-factor authentication setup.
- Select the check box next to the affected user.
- Under quick steps, select Disable.
To enable MFA for organizations, Exchange administrators must enable modern authentication in Exchange Online. By default, newer Exchange Online tenants have modern authentication enabled.
You can enable modern authentication for tenants as necessary. Before you enable modern authentication for your Exchange organization, take compatibilities into account. Consider that the user experience will change if MFA is enabled in your organization.
For more information, see the following websites: