The action 'Set-Mailbox', 'EmailAddresses', can't be performed on the object 'User Mailbox' because the object is being synchronized from your on-premises organization. This action should be performed on the object in your on-premises organization.

After implementing and migrating Exchange 2010 On-Premises to Office 365 I am getting this error/warning. And this is not just with editing email address of Exchange Object but there are several other error/warning I am getting while performing these kind of actions.

Here is the same error I am getting when editing the E-Mail Address:

Now Microsoft says to to keep at least One On Premises Exchange Server to manage environment when you are doing Hybrid Deployment. Now the question is, what happens when I am doing Cut Over Migration with 2000 seats. Do I need to have On Premises Exchange to manage Office 365? What if a customer with 30K seat environment wants to migrate to Office 365 and keep all messaging related thing with Office 365 and Active Directory On Premises. And none of Exchange Server On Premises, what is the option in that case? These are the questions which needs to be answered.

As per this TechNet blog also, it ask you to keep One Exchange Server On Premises.

I wonder why do I get the above error? I have asked this question by my client and the only technical answer I could give them was, it’s happening due to AD Sync between On Premises and Office 365. Another thing is, last time I checked DirSync is one way Sync, and this could be one of the reasons why it’s happening. I looked at several places but didn’t got the satisfactory answer.

Here is I am creating Room Mailbox using EAC or Office 365 Dashboard and I got the some error/warning.

One thing is, after I click on OK and save and check for the Room Mailbox, it’s present in there.
I am still looking for the answer of this behavior, that why I am getting the Sync error when editing the object.


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About the Author

Gulab Prasad

Technology Consultant & Blogger

MCSE: Exchange Server 2016, 2013
MCSE: Skype for Business Server 2015
MCSE: Azure Productivity
MCSE: Windows Server 2016
MCSA: Windows Server 2012, Office 365
MCITP: Exchange Server 2010-2007 | Lync Server 2010
VMware: vSphere 6 Foundations