If you are working with AD synchronization tools (e.g. Azure Active Directory Connect) in your environment (e.g. a hybrid Exchange one), there is a high probability that you applied a default configuration for the synchronization process. If so, among various synced AD attributes there is also msExchMailboxGuid.
You will receive an error inside of Office 365:
In such a case, assigning an Office 365 license to synced on-premises users will not result in creating mailboxes. You will be able to create an Office 365 mailbox only with Microsoft migration tools, which excludes any possibility of using third-party migration tools like Cloud Migrator
If you would like to migrate via a third-party migration tool (such as CloudMigrator), you need to either set the msExchMailboxGuid attribute to NULL or rebuild the synchronization service for on-premises users from scratch, removing the msExchMailboxGuid attribute from AD synchronization.
Keep in mind that after the mailboxes are migrated, Office 365 users are synchronized with your on-premises AD environment. Due of that, you need to manage Office 365 mailboxes (e.g. change their email addresses) through the on-premises Exchange server.
If you delete the mailboxes in your on-premises organization (or decommission the on-premises server), you will not be able to modify or delete their cloud counterparts in Office 365 unless you disable the directory synchronization.
How to (re)configure AD synchronization tools for migration to Office 365
To (re)configure your AD synchronization for migration to Office 365 via third-party software, you need to exclude the msExchMailboxGuid attribute from the syncing process. Follow the steps below.
- Open Azure AD Connect.
- Click the Configure button to proceed to the next section (Tasks/Additional tasks).
- Choose Customize synchronization options from the list and click Next.
- In the Connect to Azure AD section, provide your Azure credentials.
- In the Domain/OU Filtering step, choose Organizational Units (e.g. HybridUsers) that you want to synchronize and click Next.
- Proceed to the Azure AD Attributes step. Select I want to further limit the attributes exported to Azure AD and clear (uncheck) the msExchMailboxGuidcheck box
- Click Next to proceed to the last section (Configure). Ensure that the Start the synchronization process when the configuration completes check box is selected.
- Click Configure to start full synchronization.
After the synchronization is finished, all the synced accounts will not have their msExchMailboxGuid attributes synced anymore.