I think I found it by myself.
1) setting up a new E-Mail-Server in SC -> System Services -> E-Mail Settings
2) in SC -> Queue Management -> Queues -> [Queue] -> Numbers/Addresses
2.1) one for inbound (Sender unchecked)
2.2) one for outbound (Sender checked)
But there I could not use the same "Address" for in- and outbound, even when I can use the same E-Mail-Account.
WHY???
Best regards,
Robert