Plesk does not send mail: Connect to Milter service inet:127.0.0.1:8891: Connection refused, 451 4.7.1 Service unavailable

Symptoms

1. Error in
/var/log/maillog
:

   postfix/smtpd: warning: connect to Milter service inet:127.0.0.1:8891: Connection refused
   postfix/smtpd: NOQUEUE: milter-reject: CONNECT from 192.0.2.2 451 4.7.1 Service unavailable - try again later; proto=SMTP

2. 
/etc/postfix/main.cf is modified with a custom value for the
smtpd_milters
entry:

   # grep ^smtpd_milters /etc/postfix/main.cf
   smtpd_milters = inet:127.0.0.1:8891, inet:127.0.0.1:12768

3. No services are listening on the port mentioned in the error:

   # netstat -ntlp | grep 8891
   #

Cause

Invalid Postfix milter configuration.

Resolution

Note: if you don't have root access to Plesk server via SSH, contact your hosting provider regarding the issue

1. Connect to Plesk server via SSH

2. Remove all custom values from the
smtpd_milters
entry in /etc/postfix/main.cf. Leave only the following one (including the comma):

   smtpd_milters = , inet:127.0.0.1:12768

3. Restart mail services:

   # service postfix restart && service pc-remote restart