This scenario applies to hybrid configurations when moving mailboxes from on-premises to Office 365.
Whenever you see the error in the migration log that says “Unable to update Active Directory information for the source mailbox at the end of the move” it means that when the mailbox move completed, MRS could not disable the mailbox on the on-premises Exchange server and then RemoteMailbox-enable the user account as a cloud mailbox.
This results in two mailboxes – the original one on-premises and the new one in the cloud. However, the on-premises mailbox is inaccessible and autodiscover gets invalid information to setup the outlook profile.
To resolve this, perform these steps manually on the on-premises Exchange server in the Exchange Management Shell:
Enable-RemoteMailbox –Identity <alias> -PrimarySmtpAddress email@example.com –RemoteRoutingAddress alias@TenantName.mail.onmicrosoft.com
Wait for (or force) AD replication, then manually force a DirSync