If you are synchronising your Office 365 account with your on-premises exchange/Active Directory, you will know that you cannot edit exchange user properties using the Office 365 administrator portal. If you try, you will come across this error or a similar one:
The operation on mailbox “X” failed because it’s out of the current user’s write scope. The action ‘Set-Mailbox’, ‘EmailAddresses’, can’t be performed on the object ‘X’ because the object is being synchronized from your on-premises organization. This action should be performed on the object in your on-premises organization.
The reason for this is due to the fact that the AD and O365 are synchronised. Office 365 knows this and does not allow you to make any changes on O365 if there is a corresponding attribute that links up with your Active Directory.
In this article, I will show you how to add e-mail aliases using the Active Directory Service Interfaces Editor (adsiedit).
Go to Start > Run and type adsiedit.msc
Now, find the unit where your AD user’s reside
Right click the user you want to edit and click Properties.
Find the variable proxyAddresses – this is the one you want to edit. When you add new e-mail aliases, you want to make sure that your primary e-mail address will start with upper-case SMTP. Your aliases, aka, secondary addresses should be lower-case smtp.For example, I want my primary e-mail address to be email@example.com In the proxyAddresses attribute, I would put: SMTP:firstname.lastname@example.org As my alias, I want email@example.com… to do this, I will use lower case smtp: smtp:firstname.lastname@example.org
Apply the new settings and wait for your active directory to be synchronised with Office 365 (by default this happens every 3 hours but you can force this on your synchronisation server by following thesesteps)