Did you have your account with which you logged into Control Hub created on the AD and enabled before enabling the Directory sync. If you have already added users manually, performing an Active Directory synchronization may cause previously added use...
Is this 0365 or Exchange on-prem. Assuming this is on-prem, you may want to check with the Exchange team on the Throttling policy as documented here on Cisco site. This may be a good point to start the troubleshooting.
