If a user forgets their password and gets locked out of their account, the alternate email address is required to reset their password.
  1. Navigate to the Active Users page in the Microsoft 365 Admin Center: https://admin.microsoft.com/AdminPortal/Home#/users
  2. Select the User by name
  3. Click Add address under Alternate email address:
  4. Enter Email address:
  5. Click Save:
  6. Done! You will see this green banner when successfully added: