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Starting an on-prem to Cloud migration with a hybrid configuration. On-prem users can e-mail anyone just fine. Cloud users can e-mail externally and other cloud users, but not on-prem users. E-mails from cloud to on-prem users do not error, but rather appear to be sticking in the cloud and not be synched with on-prem. If an on-prem user logs into the cloud user interface via the tenant link, the e-mails from the cloud users are there (but they can't see the on-prem e-mail in that view).

I've looked at mail flow and user SMTP settings in general as it seems like something is disconnected, but it's kind of blind poking around. Internet searching only finds people where the mail actually errors out. In my case there's no error, it delivers, just not to the right place!

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Additional information: Trying to test the O365 to on-prem connector with on-prem users fails. It reports that the on-prem user is in O365, so can't send. However, if I inspect the mailbox it is clearly marked as on-prem.

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According to your description, if the user login Office 365 OWA, he can see the email, we could know that the mail flow is working fine. And now this issue we need to confirm is the type of mail flow, check if it is from cloud to on-premise server. How did you check the mailbox? The accounts can be synced to cloud but the mailbox cannot, you can migrate it to the cloud. In other words, there is only one mailbox, there is no sync. You can run get-user in EMS of the on-premise server, please check if it is Mailuser or UserMailbox. If it is mailuser, it means that its mailbox is not on on-prem.

  • It looks like O365 and on-prem are not in sync somehow. Each one has a separate mailbox (and contents) under our domain name. Each also believes it is the authoritative mailbox for the user. I can't even migrate mailboxes to/from O365 now as I can't select those people! Is there a way to just delete the bad mailboxes from O365 entirely? – Brian Knoblauch Dec 28 '18 at 13:03
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    Did you get any error with AAD connect? You also could see the sync issue in Office 365 portal. Based on my experience, this account has mailboxes both on premises as on Office 365, this can happen when you assign the user an Exchange Online license before the mailbox has been migrated to Office 365. To remove the mailbox in Office 365, you could refer to the following steps: brisk-it.net/deleting-office-365-mailbox-hybrid-deployment – Jayce Yang Dec 31 '18 at 7:22
  • Our domain ended up with all our e-mail boxes both in the cloud and on-prem and that totally confused the hybrid so it was unable to properly route e-mail. Hired a consultant to grind through remediating all of our e-mail boxes for us. – Brian Knoblauch Jan 18 at 19:09

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