By default Plesk on Linux uses Postfix for outgoing email, and by default listens on port 25 for outgoing SMTP mail. Some service providers do not allow to send emails on that port, and tragedy occurs: clients can’t send email with their Plesk servers. Not good.
Other SMTP ports will usually work, such as the other favourite 587 – but by default, Postfix is not listening on this port for email submissions – at least not in Plesk 12.0.8 on CentOS 7.
Here’s how to enable port 587 for such ventures:
Open the Postfix configuration file at /etc/postfix/master.cf and find the following line. It’s commented out. All we have to do is to remove the hash in front of it, and email can be sent via port 587:
submission inet n - n - - smtpd
Restart Postfix for the changes to take effect. In CentOS 5 and 6:
service postfix restart
This will also work in CentOS 7, but to be more precise:
systemctl restart postfix.service
Note that port 587 needs to be open in your firewall. If the Plesk Firewall Extension is enabled, it’ll take care of it for you automagically.