Mailbox migration failure: the provided email address was not unique on the source side.


A mailbox failed to be migrated to Exchange Online with the following error:

SourceEmailAddressNotUniquePermanentExeption: The provided email address “” was not unique on the source side.


The mailbox is being migrated from Exchange 2007 to Exchange Online via a staged migration.


An LDP search for the address (against the mail attribute) returned two results in the directory.



The duplicate address is on the second object, as it’s a CNF entry (caused by an AD object replication collision), this is highly likely to be the object where the duplicate address needs to be removed from.  However, I confirmed this by comparing the attributes of both objects.


I could just delete the second entry; however, in this case I removed the mail address from the second AD object using ADSIEdit.

Create a new connection, specifying the DN of the second object.


Edit the mail attribute on the object.



Clear the attribute.


I resumed the mailbox migration and the mailbox migrated successfully.



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