How can I set msExchMailboxGUID attribute to null?

How can I set msExchMailboxGUID attribute to null?

Issues in attempting to migrate from on-premise Exchange

When migrating from on-premise Exchange to Exchange Online, you receive the following error:

This is due to the user's msExchMailboxGUID attribute in the system pointing to the old on-premises mailbox value, after being synced from Active Directory to Azure Active Directory.


This can be set within Synchronization Rules Editor in Azure AD Connect. Run the Synchronization Rules Editor as an administrator.

Click on In from AD – User Exchange to edit the Inbound Rule Type.

Select Transformations. Find the msExchMailboxGuid attribute. Change it to the following:
Expression - msExchMailboxGuid - NULL - Checkmark Apply Once - Click on Update.

At this point, perform a full synchronization with Azure AD Connect.

Now users may be assigned an Exchange Online license, and your Migration tool may be used to migrate their mailboxes to Office 365.

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