Port forwarding red2red.

[Guest]

[sysbt]

Dear,
Due to my isp I can not use the well-known ports range.
So I'm using dyndns service to forward smtp to port 10025, which is forwarded to a endian 2.2 as email scanner (in-outbound, smart server to my isp smtp servers).
Is there a way to forward on the red interface port 10025 to port 25 (this way I could remove the internal endian 2.2 based mail scanner).
any-tcp-10025 to 25 smtp proxy on red. Or do I want something that is impossible?
kind regards,
Bruno