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/ 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.
/etc/init.d/postfix restart

IMPORTANT: Also make sure your firewall is letting incoming connections on port 587 as well.

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