To configure your AD synchronization for migration to Office 365, you need to exclude the msExchMailboxGuid attribute from the syncing process. Follow the steps below.

  1. Open Azure AD Connect to begin installation or to re-configure an existing installation
  2. Choose the Customize option to customize synchronization options
  3. In the Connect to Azure AD section, provide your Azure credentials.
  4. In the Domain/OU Filtering step, choose Organizational Units (e.g. HybridUsers, as shown in Fig. 1.) that you want to synchronize and click Next.

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Fig. 1. Selecting Organizational Units to be synchronized.

    5. Proceed to the Azure AD Attributes step. Select I want to further limit the attributes exported to Azure AD             and clear (uncheck) the msExchMailboxGuid check box, as shown in Fig. 2.

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Fig. 2. Excluding msExchMailboxGuid from synchronization.

        6. Click Next to proceed to the last section (Configure). Ensure that the Start the synchronization process when configuration completes check box is selected.

  1. Click Configure to start full synchronization.

After the synchronization is finished, all the synced accounts will not have their msExchMailboxGuid attributes synced anymore.