This blogpost describes how to add an extra email address on your O365 mailbox. Someone told me that when he tries to add an extra email address using the Office 365 admin partner he receives the following error :
The error says that this action (adding an extra email address) should be performed on the object in your on-premises organization. This means that your Active Directory is connected to Azure (O365). So extra email address should be added on the specific user properties and not directly using the Exchange admin interface. Your changes are automatically replicated to Azure.
Go to your domain controller and open dsa.msc (Active Directory Users and Computers) and go to the user you wish to provide an extra email address. Important : don’t search for this specific user, this way you don’t see all the properties later on. Also make sure that you have the advanced view enabled in dsa.msc, otherwise you won’t see the Attribute editor tab.
Open the properties of the specific user and select Attribute Editor and go to ProxyAddresses. Now you have the option to add an extra emailaddress, use the following Value : smtp:firstname.lastname@example.org
Make sure that the @email.nl domain is registred correctly in O365.
(Thanks to Roelf Z for the comment)