Done some testing over the weekend and found out that it is actually tied to the process itself.
This means that if you have process A for customer A, all the notifications can be configured to appear to be sent from A@A.com.
Similarly, you would need a seperate process flow (process B) to send from B@B.com.
This works regardless of how the ticket has been generated, whether thru email auto ticket creation or thru the service portal.
The customer has accepted this and will proceed to configure separate addresses on a per process basis.
Could you explain how this was achieved? We are trying something similar...