Mail delivery fails on Plesk Onyx server: 451 4.7.1 Service unavailable - try again later

Symptoms

Mail delivery fails on Plesk Onyx.

The following error records are found in /var/log/maillog :

CONFIG_TEXT: warning: connect to Milter service inet:127.0.0.1:12768: Connection refused
NOQUEUE: milter-reject: CONNECT from example.com[203.0.113.2]: 451 4.7.1 Service unavailable - try again later; proto=SMTP

Cause

pc-remote service is stopped

Resolution

Using Plesk interface

Go to Tools & Settings > Scheduled Tasks > Add new

Input the command one by one and click Run Now:

CONFIG_TEXT: /usr/sbin/service pc-remote start
/usr/sbin/plesk repair mail

Using SSH connection

Connect to the server via SSH.

Start the service and try to send mail again:

# service pc-remote start

If the issue persists, run the following command:

# plesk repair mail