Adding email alias in Exchange Online no longer works and throws out the below error

New Contributor
I had to disable AAD Connect recently and when I got everything back up and running, I received the below error when adding and alias and this is happening for every account I have tried. 
I do know that I can add alias to proxyaddress in attribute editor for user in AD but I want to be able to add alias in Exchange Online and not in AD. Does anybody know how to set this up as it was working that way previously?
An Azure Active Directory call was made to keep object in sync between Azure Active Directory and Exchange Online. However, it failed. Detailed error message: Unable to update the specified properties for on-premises mastered Directory Sync objects or objects currently undergoing migration. The issue may be transient and please retry a couple of minutes later. If issue persists, please see exception members for more information.
2 Replies

If you have dirsync enabled, you cannot make such changes directly in ExO. Your on-premises AD is the "master" and O365 admin interfaces will always throw a similar error.


The only workaround for this scenario is to use the -WindowsEmailAddress parameter:


Set-Mailbox -WindowsEmailAddress


which will set as the new primary SMTP address, while keeping the old one as secondary. This still does not guarantee that the change will not be overwritten during the next full sync cycle, moreover it's not an officially supported mehtod.

@Vasil Michev 


Hi Vasil,


I thought that might be the answer - thanks for coming back to me.