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.

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.

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.

Edit the mail attribute on the object.

Clear the attribute.

I resumed the mailbox migration and the mailbox migrated successfully.