I recently came across this problem and after searching for hours I still did not get the entire solution in one thread. I eventually resolved the problem using other information picked up from other threads within the Linux Questions site.
I thought i would make it easier to find in one place by posting my solution here.
After setting up Postfix and configuring the config files, when trying to access the server on port 25 by telnet to the localhost (127.0.0.1) we gain access to the server. If you telnet to another interface on the same machine for example 192.168.30.x on port 25 you get "Connection refused".
Edit file - /etc/postfix/main.cf
From - #inet_interfaces = all
To - inet_interfaces = all
From - #mynetworks_style = subnet
To - mynetworks_style = subnet
From - mynetworks = 127.0.0.0/8
To - mynetworks = 192.168.30.0/24, 127.0.0.0/8
Edit file - /etc/postfix/master.cf
From - smtp inet n - n - - smtpd
To - 0.0.0.0:smtp inet n - n - - smtpd
Then restart the Postfix server:
I hope this helps.