Some internet access providers have port 25 disabled in their routers to prevent spam. If you run your own mailserver in a datacenter, you might have to enable the submission port (587) in postfix to be able to send emails from your local email client to your own mailserver.
To enable port 587, edit the file /etc/postfix/master.cf
and remove the # in front of the line:
#submission inet n - n - - smtpd
so that it looks like this:
submission inet n - n - - smtpd
and restart postfix: