In Exchange on-premise migration to Office 365, one of the first steps is to create the users in Office 365.
Most of the time, the Microsoft Azure Active Directory Connect tool is used. The default configuration of this tool cause problem if an Exchange server exists (for example, in an Exchange hybrid environment). This problem is due to the msExchMailboxGuid attribute.
By default, the user synchronization will cause this attribute to be synchronized to Office 365 and therefore Exchange Online. This is a problem because Exchange Online will assume that the mailbox is not migrated and will report the following error when assigning an Exchange Online license:
This user’s on-premises mailbox hasn’t been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed.
Exchange Online recognizes that the user has a mailbox ID and expects the mailbox to be migrated with the native Microsoft tool (and only this tool).
From the Azure AD Connect server, execute Synchronization Rules Editor.
Click on In from AD – User Exchange > Edit.
Click on Yes. This will create a new rule and disable the default rule. This is good practice because all default rules are changed each time Azure AD Connect is updated.
- Name: modify if needed
- Description: modify if needed
choose a number below than 100 for the rule to be executed before the default rules
- Unchecheck Disabled
Click on Transformations and modify the line related to msExchMailboxGUID :
- FlowType : Expression
- TargetAttribute : msExchMailboxGUID
- Source : NULL
- Check Apply Once
- Merge Type : Update
- Click on Save.
Finally, you must refresh the Azure AD Connect database to apply the rules change : Open Azure AD Sync Service. In the Connectors tab, right click on the AD connector AD domain.com > Run > Full Synchronization.