Your user ID is assigned to you by your work or school. It looks like ellen@contoso.onmicrosoft.com, ellen@contoso.edu, or ellen@contoso.com. This work or school account is the email address and account that you use to sign in to Microsoft 365 for business for work or school.
: If your organization is also signed up for Windows Azure, CRM, or you've added your domain to any of our online services, you should use the same user ID and domain to sign in, and for access to all your services.
Your work or school account is different from your Microsoft account. Your Microsoft account is the one that you use for personal services like Xbox Live, Outlook.com, Windows 8, Windows Phone, and more. Check out this video, What is a Microsoft account? for more details.
You can't use your Microsoft account to sign in to Microsoft 365 for business for work or school, unless someone in your organization shared a specific document with you via your Microsoft account. Then you could access that document using your Microsoft account.
When you get your work or school account for the first time, you'll also get a temporary password to sign in with. When you sign in, you'll be asked to create your own password. Use it with your user ID every time you sign in to Microsoft 365 for business. For more information about changing your password, including how to create a strong password, see I forgot the username or password for the account I use with Office.
Microsoft 365 for business admins
The following information is for Microsoft 365 for business admins only.
Create new user accounts
You’ll create a new user ID, with a unique user name, for each user account that you add to your subscription. The domain part of the user ID is either the onmicrosoft.com domain that you chose when you signed up for Microsoft 365 for business or another domain—for example, your organization’s custom domain—that you’ve added to Microsoft 365 for business.
For example, if you chose fourthcoffee.onmicrosoft.com as a domain name when you created your account, and you’re creating a user account with the user name colin, the user ID would be colin@fourthcoffee.onmicrosoft.com.
If you want to use your custom domain for user IDs and email addresses, see Verify your domain in Microsoft 365.
Set up and use an alternate email address
When you sign up for Microsoft 365 for business, you also need to provide an alternate email address that’s not connected to Microsoft 365 for business. This email address is used for important notifications, such as resetting your admin password, so you must be able to access the email account if you don’t have access to Microsoft 365 for business. After you complete the sign-up process, you’ll get an introductory email at your alternate email address that includes your user ID and the URL of the page where you will sign in to Microsoft 365 for business. If you forget your user ID, or if you don’t remember the Microsoft 365 for business URL, look at the introductory email for this information.
: The alternate email address you are asked to provide during the sign-up process is used for important communications, like when resetting your admin password. Any product-related communications that you choose to receive from Microsoft 365 for business are also sent to this email address unless you change your contact preferences. You can update your contact information on the Me page. For more information about changing your contact preferences, see Change your contact preferences.
