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

Issue

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

SourceEmailAddressNotUniquePermanentExeption: The provided email address “test.user1@x500.co.uk” was not unique on the source side.

SourceAddressNotUnique

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

Investigation

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

LDPDuplicateSearch1

LDPDuplicateSearch2

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.

Resolution

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.

ADSIEditDuplicate0

Edit the mail attribute on the object.

ADSIEditDuplicate1

ADSIEditDuplicate2

Clear the attribute.

ADSIEditDuplicate3

I resumed the mailbox migration and the mailbox migrated successfully.

 

 

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