If you want your hosted email or other services to use a domain that you own rather than the one you were given at signup, you can add the domain to Microsoft Office 365. For example, you might want to use an email address like mike@fourthcoffee.com instead of mike@fourthcoffee.onmicrosoft.com.

Plan your domain strategy if you're just getting started

Switching to another Office 365 plan

Understanding domains and DNS records management

How your Office 365 domain relates to things besides email

Troubleshooting issues

More about domains in Office 365