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'.
Set-Mailbox <username>@contoso.com -ImListMigrationCompleted $false
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.
If you experience issues when you move mailboxes to Exchange Online in Office 365, you can run the Troubleshoot Office 365 Mailbox Migration tool. This diagnostic is an automated troubleshooting tool. If you're experiencing a known issue, you receive a message that states what went wrong. The message includes a link to an article that contains the solution. Currently, the tool is supported only in Internet Explorer.
Still need help? Go to Microsoft Community or the Exchange TechNet Forums.
Article ID: 2932443 - Last Review: Dec 29, 2016 - Revision: 1