Hybrid - Mail Users Missing in O365

New Contributor

Hi - I'm working on a migration from Exchange 2010 -> O365 and must be missing something. I've configured AADC to sync the users and I'm successfully seeing them in O365. I've only licensed the 1 user who I actually migrated for testing. However, I would expect that the other users would be showing up as Mail Users. 


I've verified the following:

  • Address Book Policy is enforced
  • Proxy Addresses contains the @onmicrosoft.com address
  • User isn't (and hasn't been) licensed in O365
  • Target Address is not set
  • Exchange Mailbox GUID exists on account


What am I missing? One other thing that might help point in the right direction is that I successfully completed a Move Migration for one mailbox and now that mailbox is completely gone from On-Prem. I would expect to see it as a Remote Mailbox.




4 Replies

Not really sure what you mean by the "other users" part. As for the user you migrated, can you check the status in the O365 admin center? I've seen few similar threads recently, which got me thinking there might be some issue on MS side.

@jweier_elys if you have Hybrid configured correctly, it should work as follows:

- You use Exchange on prem for all mailbox management, on-prem and online, including mailbox creation, modifying proxy addresses etc.

- You will see on-prem and online mailboxes in Exchange on-prem. The online ones will show as Office 365 mailboxes.

- You will online see online ones in Exhange online.

- The address list is synced, so users will be able to see everyone in the address book, if they are migrated or not.

- If configured correctly, it doesn't matter if you license users for Exchange online before they are migrated. This is because it knows if the users have a mailbox on prem or not through AD attribute sync. For this to work, you MUST have configured Exchange hybrid in Azure AD Connect. If you have not done that and add a license, you end up with 2 mailboxes, one on prem and one online, and a right mess.


If you migrate a mailbox and you don't see it on-prem any more, you have an issue, and I suspect you have not configured hybrid mode correctly, or enable Exchange hybid in AD connect.


@CloudHal Thank you for your reply. The users I synced with AADC connect but didn't move yet eventually showed up as Mail Users. It took a handful of hours for them to show up as Mail Users after they were initially synced.

Seems like it might've been a combination of my impatience and O365 being a little slow. After a few hours the users that were synced via AADC but not yet moved showed up as Mail Users.