I'm missing a bit here. The Proxy will route incomming email on port 25 to my MTA receiving on port xyz.
The MTA will be able to send out on port 25 again.
Why would I need to relay back to the Proxy?
In postfix, relayhost can be set. But my understanding of this setting is that email is sent via i.e. an ISP mailhost..

Thanks for your help!