I agree that Postfix may only be listening on the loopback interface.
To check, run the following commands (and post the output for us to see):
netstat -pantu|grep -w 25
lsof -i :25
I would try the suggestion(s) offered by the previous user.
If it doesn't work, tell us whether you're connecting to your postfix server via it's hostname of ip address.
Another thing to try is to run tcpdump to see if any TCP/25 packets are hitting the postfix box from your LAN.
You can do that as follows (assuming you LAN is on eth1):
tcpdump -i eth1 port 25 -A