To allow the sending of messages on the mail server without encryption via port 25, you need to open the Postfix configuration file, for example, in the nano editor (in which Ctrl+X to exit, y/n to save or cancel changes):
sudo nano /etc/postfix/main.cf
And uncomment two lines:
smtpd_sasl_auth_enable = yes smtpd_sasl_security_options = noanonymous
And leave the commented line:
Reload Postfix to apply the changes:
sudo service postfix restart
After that it will be possible to send messages via SMTP through port 25 without SSL, and as usual through 587 with SSL.