Cugar15 Wrote:
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> 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.

The MTA needs to be aware of the proxy (since I think your relay denied message is because of this) and if so it is masking your main problem, see here for a bit more details http://blog.haproxy.com/2012/06/30/efficient-smtp-relay-infrastructure-with-postfix-and-load-balancers/

nginx is no different here in its smtp use.

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nginx for Windows http://nginx-win.ecsds.eu/