I'm not certain is this will fix your problem, but I've been looking around google trying to find a fix for postfix not listening on port 25 when accessed from another computer (external). Here's how I fixed it on mine.
When trying to telnet to port 25 the error message was: "Could not open connection to the host, on port 25: Connect Failed". I noticed that I could telnet from the host machine to localhost or 127.0.0.1 and postfix displayed the welcome message fine.
I found out that my server was only listening locally to port 25 using "netstat -an | less" displaying "127.0.0.1:25 LISTEN" instead of "0.0.0.0:25 LISTEN". I did some searching and found out that this was because of postfix running smtp in chmod mode (whatever that means). Looks like Manrake 10 installs with this by default for security.
To fix this, I changed the postfix "master.cf": vi /etc/postfix/master.cf
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (yes) (never) (100)
smtp inet n - y - - smtpd
I just changed the "y" to a "n", reboot the server (postfix reload didn't seem to work). I was then able to telnet to my server on port 25!
Hope this help out a few people,