A mailbox failed to be migrated to Exchange Online with the following error:
SourceEmailAddressNotUniquePermanentExeption: The provided email address “firstname.lastname@example.org” 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.