I have an AD forest with 1 parent domain and 3 subdomains(A.lan, 1.A.lan, 2.A.lan, 3.A.lan). Azure AD Sync is configured and all accounts are synchronized. All domains have differents supplementary UPNs which are used for email domains. (w.com, x.com, y.com, z.com)
My Exchange setup consist of 2 CAS servers and 3 MBX servers. Hybrid configuration is setup. I have migrated all users from the parent domain to Office 365 and finalized the migration. MX and autodiscover now points to Office 365 records. In my on premise Exchange I have changed the accepted domains list to set my primary domain UPN as an external redirect.
I have an issue with post migration created users. If the user exists in the on premise exchange and Office 365, mail will be delivered in the on premise mailbox if coming from the non migrated domains. If email comes from the migrated domain, it is delivred on the Office 365 MBX.
If the user only exist in Office 365 (this is what I want), then no issue if the email is sent from a migarted domain MBX but is not delivered (since it doesn't exist) if coming from a not migrated domain.
I though setting the migrated domain as an external redirect would be enough to tell Exchange it is not authoritative anymore.