When using the directory synchronization tool with Office 365, configure the directory synchronization tool to provision the accounts within Office 365 a specific way.

By default, when synchronizing a user from an on-premises environment that has a mailbox assigned to it, assigning an Office 365 license will not result in the creation of a mailbox. The mailbox is created when initiating the free Microsoft migration tools. MigrationWiz can only migrate to mailboxes that already exist.

We can alter the directory synchronization behavior such that Office 365 license assignment results in the creation of a mailbox without initiating the Microsoft migration tools. You can do this by performing the following:

1. Activate, download and install the directory synchronization tool.
2. Remove the synchronization of the attribute msExchMailboxGuid to Office 365.
1. Open Windows Explorer.
2. Navigate to C:\Program Files\Microsoft Online Directory Sync\SYNCBUS\UIShell.
3. Launch the application miisclient.exe.
4. Click on the Management Agents tab at the top.
5. Right-click on the Management Agent TargetWebService, and select Properties.
6. From the navigation pane, click on Configure Attribute Flow.
7. In the Configure Attribute Flow section, expand the node Object Type: user.
8. Locate and select the mapping for the attribute msExchMailboxGuid.           

    

9. Click on the Delete button which will remove the selected mapping.
10. Click on OK to save the changes.
3. Verify that no object have synchronized to Office 365. If objects have synchronized to Office 365, remove all synchronized objects:
1. Open your browser to the Microsoft Office 365 Administrative Portal.
2. Click on Users in the navigation pane.
3. If you have users synchronized into Office 365, you will need to remove those users and resynchronize them as they were synchronized with the msExchMailboxGuid attribute.
4. Force directory synchronization.

You are now ready for migration. To migrate a mailbox, perform the following steps:

1. Activate a synchronized user and assign a license.
2. Set mailbox forwarding on the Source Exchange mailbox.
3. Initiate migration with MigrationWiz.