Some internet access providers have port 25 disabled in their routers to prevent spam. If you run your own email server in a data center, 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 mail server.
Enable Submission Port 587 in Postfix
To enable port 587, edit the file /etc/postfix/master.cf
nano /etc/postfix/master.cf
and remove the # in front of the line:
# submission inet n - y - - smtpd
so that it looks like this:
submission inet n - y - - smtpd
and restart postfix:
systemctl restart postfix