To add an alias to a mailbox in an Office 365 environment whose Active Directory is synchronized, you must use the local directory (On Premise) to add the alias in the proxyaddresses attribute.
4. Enter the alias to add (smtp:[email protected]) 1 and click on Add 2 .
For information the address written with SMTP is the main email address.
6. Apply the properties of the user.
7. The alias will be added during the next synchronization cycle.