Kayako will use your default email address to send any messages generated by either the notification center or an automation you've built. You can change your default email address from the admin area by clicking on Email in the sidebar. Hover over the address you want to change to, and then click the Make default option that appears:
How do I change my default email address?
PLAN AVAILABILITYNOTE: If you're not sure what plan you're on, sign into the admin area and check the 'Plan' page.