MicrosoftDanielNiccoli After Exchange hybrid deployment, if an organization plan to keep AADConnect synchronization, all Exchange Online modifications cannot be perform throught Office 365 admin portal but from the source authority (based on-prem). So, recommandation is to keep on last Exchange server on-prem for management purpose.
At the last Ignite, there were rumors about a new way that allow to keep AADConnect and be able to manage Exchange attribute online but no announcement was made... I was hooping a good news for this year on this topic 🙂
