When creating a new user in Office365 with a hybrid or active directory syn’d setup it is quite common for Office365 to append the .onmicrosoft.com domain to any new address you set up.
This unfortunately cannot be changed by means of GUI, you need to change the username in O365 using powershell.
To change the username in Office 365:
1. Make sure you have the WAAD module
2. Run PowerShell
3. Type “Connect-MsolService” –> ENTER. Enter your admin credentials for Office 365 –> OK
4. Use the below command to update the primary email address used to log in exchanging parts in bold;
"Set-MsolUserPrincipalName -UserPrincipalName current_email@****.onmicrosoft.com –NewUserPrincipalName new_email@domain"
Nice and easy 🙂