Problems in Portforwarding SMTP to internal server

[stian@barmen.nu]

I use this command in my IP-tables script to forward the smtp port of an internal NAT'ed server to the external firewall. Here is the command:

iptables -t nat -A PREROUTING -i ${UPLINK} -p tcp --destination-port 25 -j DNAT --to 10.26.12.11:25

To forward ports like 80, 143 and others this works without a problem. There seems to be some strange problem with port 25 on the firewall. If I bind the service to port 26 on the firewall it works...! Ergo this command works:

iptables -t nat -A PREROUTING -i ${UPLINK} -p tcp --destination-port 26 -j DNAT --to 10.26.12.11:25

Any ideas why I cannot bind to port 25? I am not running any smtp service, just ssmtp wich does not utilize a smtp service.

Hope somenone can help!
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Best regards
Stian B. Barmen