This can be set within Synchronization Rules Editor.


  • This is not supported, if you have performed a directory synchronization using DirSync.
  • It is supported, if performing a directory synchronization using AAD Sync, or AAD Connect.
  • For more details on the correct order of steps to follow for directory synchronization, refer to KB004336.


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


Enable AADSync or AAD Connect and perform a full synchronization.

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