The log does not contain any connections from a external mailserver, so the problem can be:
1) The dns record is not corrcet.
2) Port 25 is not forwarded from router to the server IP if your server is installed in a local network.
3) Port 25 is blocked by a firewall on the server, a router or by your ISP.
4) Postfix is not listening on the external network interface.
Please post the output of: