Change your default domain for email in Office 365
After you set up a custom domain in Office 365 (like fourthcoffee.com, for example), you can change the default domain used for new email addresses when you add users in Office 365.
If you already have a custom domain and want to change your default domain to a different one, just go to Domains > Add domain to follow the same step-by-step wizard in Office 365 to add another domain. Then change your default domain as shown below.
You must have at least one custom domain that you've added to Office 365 before you can choose a default domain.
To choose a new default domain for email addresses: