Your user ID is the unique email address that was created for you to use when you sign in to Office 365. A user ID looks similar to the following: firstname.lastname@example.org or email@example.com. Your user ID lets you sign in to Office 365.
When your user ID is created, Office 365 assigns you a temporary password that is sent to you so you can sign in to Office 365. When you do so for the first time, you have to create your own password. Use it with your user ID every time you sign in to Office 365. For more information about changing your password, including how to create a strong password, see Change your password.
Office 365 admins
The following information is for Office 365 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 Office 365 or another domain—for example, your organization’s custom domain—that you’ve added to Office 365.
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 firstname.lastname@example.org.
If you want to use your custom domain for user IDs and email addresses, see:
Set up and use an alternate email address
When you sign up for Office 365, you also need to provide an alternate email address that’s not connected to Office 365. 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 Office 365. 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 Office 365. If you forget your user ID, or if you don’t remember the Office 365 URL, look at the introductory email for this information.
Note 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 Office 365 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.