Exchange Web Services (EWS) reported that a mailbox cannot be accessed because of throttling limits imposed by Exchange.
Check the error to determine if it was reported for the Source or Destination system.
If migrating from or to Office 365, enable impersonation to reduce the effect of throttling. See Impersonation and Delegation.
If migrating from or to Exchange 2010 or later, and you have control over the email system, disable the Exchange throttle policy. See Disable throttling policy on Exchange.