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.