Outlook limits its choices of authentication schemes to schemes that are supported by RPC. After you enter your credentials, they're transmitted to Office 365 instead of to a token. In these scenarios, you're prompted for credentials, and Outlook doesn't use Modern Authentication to connect to Office 365. You migrate your mailbox to Office 365 from an Exchange server that Outlook connects to by using RPC. Microsoft Outlook connects to your primary mailbox in an on-premises Exchange server by using RPC, and it also connects to another mailbox that's located in Office 365. Applies to: Outlook 2016, Outlook 2013, Exchange Online, Outlook for Office 365, Outlook 2019.Outlook prompts for password and doesn't use Modern Authentication to connect to Office 365
