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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.

Thank you.

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    No user should have a mailbox in both Exchange local and Exchange online. You are going to have to decide what mailbox you want to keep and remove the other one. If the mailbox is on Exchange online you need to have targetAddress set to user@tenant.mail.onmicrosoft.com and that same address needs to be in the proxyAddresses. Also you really should say what version of Exchange local you are on. As well the user does exist both places, but the mailbox should only exist in one. Exchange local will have a remote mailbox set for the Exchange online user. – JBaldridge Aug 4 '17 at 16:24
  • @JBaldridge should make this an answer and not a statement, as it is correct, a user should only have a single mailbox in this scenario, having it in both places will lead to this behavior (and it would be expected because of how Exchange works). – Jesus Shelby Dec 16 '17 at 22:55
  • @Jesus Shelby -- I have posted that as an answer. Thank you. At the time I thought there might be some back and forth to get a more complete answer to post and that is why I did it that way. – JBaldridge Dec 18 '17 at 16:12
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No user should have a mailbox in both Exchange local and Exchange online. You are going to have to decide what mailbox you want to keep and remove the other one. If the mailbox is on Exchange online you need to have targetAddress set to user@tenant.mail.onmicrosoft.com and that same address needs to be in the proxyAddresses. As well the user does exist both places, but the mailbox should only exist in one. Exchange local will have a remote mailbox set for the Exchange online user.

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