Customer has accounts for external partners which are synced on a 365 registered domain. So accounts appear in Azure AD.
We want to add them to an external network.
The accounts have the partners email address attached to them. There are no mailboxes in 365 for these accounts.
Inviting them into an external network fails as the invites never arrive in the partners own email mailbox.
Sending invites directly to the partners email address works, but we want them to sign in with their partner provided ID and not their email address. This is just part of the customers governance where they control the login identity.