When migrating from Exchange on-premises to Office 365 with a Third-Party tool such as Migration Wiz from BitTitan you need to remove the msExchMailboxGuid from the synchronised attributes otherwise you will get the following error:
This is because Exchange Online recognises that the msExchMailboxGUID attached to the user is an Exchange on-premises mailbox, and so will not provision a new mailbox and therefore break mail flow. But in a migration scenario you would want to run the two mailboxes side by side, i.e. if you have slow comms and want to “front-fill” the mailbox ahead of the migration date.
In this case you would need to edit your synchronisation option in AAD Sync. To do this follow these steps:
- Open the Synchonization Service
- Under connectors right-click your on-premises Active Directory connector and click Properties.
- Under Select Attributes you need to untick msExchMailboxGUID and click OK.
Once you have completed the above, kick off a sync and you will then be able to license your users with the original error!