Can Azure AD be configured to use a 3rd-party IdP for some users, even when the domains of those users are not known?

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I want an app to use Azure AD (AAD) for authentication and user info, but unbeknownst to the app, AAD will not have a record of some of those users but will know an IdP it can ask (federate) about those users. The other IdP is not a Microsoft product. The other IdP already manages hundreds or thousands of users, so it is not practical to "invite" all those users to AAD. We don't know all the email addresses for all those users and those users come from many different "domains" (if you base that off their email addresses). I just want to set up a trust of the 3rd-party IdP by AAD and then have AAD pass on the appropriate tokens to the app. Can it be done?

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Vlad DX On

Looks like you are looking for a formerly-known "Direct Federation". How it's called "SAML/WS-Fed identity provider (IdP) federation".

When you set up federation with a partner's IdP, new guest users from that domain can use their own IdP-managed organizational account to sign in to your Azure AD tenant and start collaborating with you. There's no need for the guest user to create a separate Azure AD account.

https://learn.microsoft.com/en-us/azure/active-directory/external-identities/direct-federation

It is not possible to configure a "3rd-party IdP for some users, even when the domains of those users are not known?".

Domain is not just an address, there is an IdP behind the domain name (in terms of user accounts).

"Federation" == "trust". To establish trust between Azure AD and an Identity Provider, you need to explicitly configure it.

So, it is not possible to do at all.