Assume that you have an Exchange hybrid deployment that includes on-premises Exchange 2010 servers. When you try to offboard or move a mailbox from Exchange Online to an Exchange 2010 server in your on-premises environment, you receive the following error message:
You shouldn’t migrate mailbox ‘<username>’ to Exchange 2010 or an earlier version while the user’s Instant Messaging contact list is stored in Exchange. If you do, the user could permanently lose access to their Instant Messaging contact list, which will cause serious data loss. The Exchange copy might be the only copy of this user’s contact list. To continue, please contact your Instant Messaging administrator and make sure that the user’s contact list is moved back to the Instant Messaging server. After this has been done, you should be able to complete this migration. If you must migrate the mailbox despite the potential data loss, you can do so by running ’Set-UMMailbox mailboxID -ImListMigrationCompleted $false’.
However, when you run the Set-UMMailbox mailboxID -ImListMigrationCompleted $false command as stated in the error message, you receive the following error message:
The Mailbox ‘<username>@contoso.com’ isn’t enabled for unified messaging.
This problem occurs if the Lync 2013 contacts of the user who is associated with the mailbox are stored in the unified contact store in Exchange. By default, if the user signed in to Lync 2013, the unified contact store is enabled for that user. Additionally, the user’s Lync contacts are migrated from the Lync server to Exchange.
Use the Set-Mailbox cmdlet together with the ImListMigrationCompleted parameter instead of the Set-UMMailbox cmdlet. For example, run the following command:
Set-Mailbox <username>@contoso.com -ImListMigrationCompleted $false
After you run the command, perform the mailbox move request.
Note The $false setting in the ImListMigrationCompletedparameter indicates that the user’s contacts haven’t been migrated to Lync to preserve the contact list. Be aware that the solution in this section may result in data loss. Exchange 2010 doesn’t support the unified contact store feature in Lync 2013. Therefore, if you move a mailbox back to Exchange 2010 while the user’s Lync contacts are stored in the unified contact store, the user could lose access to his or her Lync contacts. You should first make sure that the user’s Lync contacts are moved back to Lync server before you move the mailbox to Exchange 2010.
Source: Microsoft TechNet