ISPs have started blocking outbound port 25, the default smtp server-to-server port. So here is a fix. Obviously your target postfix server should be listening on port 2525 for this to work. To test that, run: telnet server.com 2525 to see if you can connect. If you can connect, then do the following:
nano /etc/postfix/transport
mail.an smtp:mail.an:2525
sudo postmap /etc/postfix/transport
nano /etc/postfix/main.cf
transport_maps = hash:/etc/postfix/transport
sudo service postfix restart
