In a Microsoft Exchange Server 2010 hybrid environment, you try to on-board or off-board a mailbox through Exchange Online. If you use Exchange Server 2010 as the MRSProxy service, the migration ends unexpectedly and errors are reported.
When you on-board a mailbox to Exchange Online, the status is displayed as CompletedWithWarnings. This results in the mailbox existing in both locations.
When you off-board a mailbox from Exchange Online, the move status is displayed as Failed because the migration was unable to convert the target MailUser to a mailbox.
Additionally, the following error is logged in the move report:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.