Additional Emails for a Single User in Office365

 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.