Admin - Settings - My Settings - Email - Sending - Send from domain default value
You can change the default first half of the sender address in the system by modifying the setting above.
This will only change the first half of the sending address and not alter the domain setup within the system.
You may wish to do this if you would like all sent Campaigns from an account to appear to have been sent from another member of your team or department.
By default the username for your account will be used to send the email Campaigns however this can be changed on a per Campaign basis when you are building the Campaign.
Any changes to your dedicated sending domain need to be organised by contacting your account manager.