All Linux ports below 1024 are restricted, which means programs cannot bind those ports without being the root account. This is expected Linux behavior.
With Windows, you can work directly against port 25 without the restrictions. The recommended approach is to reroute port traffic and setup IP tables.
To reroute port traffic and setup IP tables:
For more references on IP tables, see:
http://www.frozentux.net/documents/iptables-tutorial/
https://help.ubuntu.com/community/IptablesHowTo
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