is there a chance to configure windows’ firewall to redirect data sent from an external interface to port 25 to our port 10025?
Windows built-in firewall does not support port redirecting services.
does anyone have an idea how to solve this EXCEPT using custom LSP/TDI/NDIS drivers?
Regretfully there is no other way. You can use WinpkFilter library (http://www.ntkernel.com/w&p.php?id=7) to implement port redirector solution though.