About your initial onmicrosoft.com domain in Office 365
Office 365 creates a domain for you, like contoso.onmicrosoft.com, when you sign up with the service. The user ID that you create when you sign up includes the domain, something like email@example.com. You can keep using this domain for your user ID and for other users that you add to your subscription. Some people do this while they’re using a trial version.
You have two initial domains: onmicrosoft.com and public.sharepoint.com
Your Office 365 subscription actually has two initial domains: the onmicrosoft.com domain, and a SharePoint Online domain.
You can use your onmicrosoft.com domain (like fourthcoffee.onmicrosoft.com) with most Office 365 services except with your public website.
For example, you can use the onmicrosoft.com domain with Office 365 email addresses (they are also your user IDs for Office 365), team sites, and Lync.
Your Office 365 public website is hosted on your initial SharePoint Online domain (like fourthcoffee-public.sharepoint.com) instead of on the onmicrosoft.com domain that you use with other services.
Even after adding one or more of your own domain names to Office 365, you can keep using your onmicrosoft.com and public.sharepoint.com domains.
Note You can’t rename your initial domains after sign-up. For example, if the initial domain you chose when you signed up for Office 365 was fourthcoffee.onmicrosoft.com, you can’t change it to be fabrikam.onmicrosoft.com. To use a different onmicrosoft.com domain, you’d have to open a new subscription with Office 365.
Benefits of adding your own domain
When people are ready to use Office 365 for their business, most would rather have email (and a website) that includes their own domain, like fourthcoffee.com.
To be clear, there’s no requirement to add your own domain. You can keep using the onmicrosoft.com domain with Office 365 and never switch. But using your own domain lets you have an easy-to-remember Office 365 email address (it’s also your user ID) and website URL for your business.
Say you decide to add your custom domain to the Small Business version of Office 365. After you finish, when you look in your list of domains in Office 365, the domain you added is included, like fourthcoffee.com. The initial domain is there, too (the one that includes onmicrosoft.com).
You can use your domain for several things in Office 365. For example:
Use your domain with email addresses. You can change users’ email addresses to use the more familiar domain name.
Use your domain with your public website. You can set up your public website to use your custom domain. The specific steps for this depend on whether you’re using an Enterprise or Midsize Business or a Small Business version of Office 365.
I can’t remove the onmicrosoft.com domain
The initial Office 365 domains cannot be removed. We need to keep them around because Office 365 continues to use them behind the scenes for your subscription. But you don’t have to use those domains yourself after you’ve added your own domain.
However, there are situations in which you might want to use your onmicrosoft.com domain after you add your domain.
For example, you sign up for another Office 365 subscription, perhaps because you need a different set of features. If you want to use your custom domain with the new subscription, you have to first remove it from the Office 365 subscription you had already added it to. You can’t remove domains that are being used by anything in your subscription; for example, in a user ID (an email address).
To free up the domain so you can remove it (to add to your other subscription), you could first change your user ID back to the onmicrosoft.com domain. Follow the same steps described above for editing user email addresses to use your domain. But this time, choose the onmicrosoft.com domain in the drop-down list. Follow the same steps for anyone else on your subscription who also uses the domain with email. When you’re done (assuming that you aren’t also using the domain with your website or other services), remove it from one subscription, and then set it up with the new one.
Note If you had also set up your domain with your SharePoint Online public website, then before you can remove the domain, you first have to set the website’s URL back to the initial domain. The steps to do this are different, depending on which type of Office 365 plan you are subscribed to (Small Business, or any other plan).