MAGE - Migration to Office 365 (0365) fails with System.Management.Automation.RemoteException: ######GUID##### isn't a mailbox user (4255008)

MAGE - Migration to Office 365 (0365) fails with System.Management.Automation.RemoteException:  ######GUID##### isn't a mailbox user

Error from Mage log:
""System.Management.Automation.RemoteException: 0eb397f9-2856-4ecb-a294-86f80d612009 isn't a mailbox user. at Dell.MigrationManagerForExchange.General.PwrShell.PSInternal.ExamResults()
at Dell.MigrationManagerForExchange.General.PwrShell.PSInternal.RunCommandInternal(String command, PSParam[] list)
at Dell.MigrationManagerForExchange.General.PwrShell.PSConnectorBase.RunCommand(String command, PSParam[] list)
at Dell.MigrationManagerForExchange.General.PwrShell.PSConnectorExchange.GetMailbox(String identity)
at Quest.MigrationManager.Exchange.Agent.Jobs.TargetMailbox.get_MailboxFields()
at Quest.MigrationManager.Exchange.Agent.Jobs.TargetMailbox.get_Ledn()
at Dell.MigrationManagerForExchange.Shared.Heart.StatisticHelper.GetMailboxStatistic(IMailbox mailbox, IPsExchangeConnector conn)
at Quest.MigrationManager.Exchange.Agent.Jobs.O365Job.GatherSizeInfo(PersistentStatItem item)"