Migrating On-Prem Users To Existing O365 Tenant With AD Connect Sync Already In Place

Let’s say you are in the process of migrating a client of yours to Office 365 from their on-premises environment where they are running Microsoft Exchange server.

Normally, you would create a new tenant on O365 and then create the users on cloud only to be begin with. After creating users, you would start mail migration using some 3rd party tool or PST imports. Once mail has been all ingested in cloud mailboxes you would set up AD Connect on their domain controller (Ideally) and set up the sync (make sure all PSMTPs / UPNs / Proxy Addresses matches cloud UPN). Once setup and AD sync have run, you would see that on O365 mailboxes type would change from “InCloud” to “Synced with Active Directory”.

But in some scenarios, tenant would already exist on O365 and users had already synced using AD Connect.  For e.g. you can set up an O365 tenant for licensing purposes i.e. after syncing users from AD, you would assign licenses so that outlook suite could be downloaded or installed on users’ PCs. So they are not using O365 for email but just for licensing purposes.

But let’s assume that the client decided to move to O365 for mail as well. That’s exactly the situation I ran into. Since tenant was already on O365 and users were already synced, mail migration could be started after assigning appropriate licenses to them. Normally, after assigning a license (Office 365 E3 or any other license that includes exchange online), mailbox would appear after some time as it takes time to create the mailbox. You can check that by clicking on Mail Settings in the properties of User in O365 portal. But in this scenario, it gave me the following message.

The issue here was that since tenant already existed and users were already synced so we could not add new mailboxes because these synced accounts were already associated with their on-premises exchange.

The fix was to open up Synchronization Service Manager – Connectors – Active Directory Domain Services – Properties. Untick the box next to msExchMailboxGuid. Run the sync manually and that property would be excluded from being synced and all users with appropriate licenses on O365 portal mailboxes would be created and will show up on Exchange Online portal now.

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