Follow these steps to add additional emails to a single user in Office365 or SharePoint online:
· Login to your Office365 account with an administration account
· Click on Home (top menu)
· Click on Admin (top menu)
· On the right Pane, find Outlook and select General Settings
· On the mailboxes list, select the user and click details
· On the details page, scroll down and expand E-Mail options
· Click on Add
· Enter the new email address and select the domain.
o Notice that in the list you have the onmicrosoft and perhaps a custom domain entry. If you intend to use this email with a custom domain, if this is not yet on the list, you need to first add the custom domain and then configure the additional emails.
· Click OK
The new email account should now be configured. Test it by sending emails to that account.
I hope this helps.