The following Tip is been taken from the following forum:
Some ISPs, for some strange reasons, are blocking the port 25, therefore preventing many normal email clients software from picking-up their emails from other mail servers than their own ISP’s server.
Making sure Postfix on your mail server listens on both ports 25 and 587.
Open /etc/postfix/master.cf and add the following line:
587 inet n - - - - smtpd
Activate(uncomment) the following line (if it exists):
submission inet n - - - - smtpd
Restart postfix and your done.
IMPORTANT: Also make sure your firewall is letting incoming connections on port 587 as well.