1. Login to the Office 365 Admin Portal as an administrator.
2. Click the Admin drop down menu in upper right and select Exchange.
3. On the Exchange Admin Center recipients page, ensure mailbox is selected in the top menu and double click on the user you would like to edit.
4. In the pop-up window, click email address on the left menu, then click the Add (+) icon to add a new alias email address. (Note that all of the alias (SMTP) addresses associated with the user are found in the list. To edit or delete an existing alias (SMTP) address, click the respective Edit or Delete icon and follow the prompts.)
5. In the new email address pop-up window, ensure the Email address type is set as SMTP, type the alias email address, choose the reply address preference, and then click OK. (Note that if the Make this the reply address box is selected, the login username will not change.)
6. Now that the alias email address has been added to the list, click save.