Article ID: 120384, created on Feb 28, 2014, last review on Oct 12, 2016

  • Applies to:
  • Plesk 12.5 for Linux
  • Plesk 12.0 for Linux
  • Plesk 11.0 for Linux
  • Plesk 11.5 for Linux


Mail delivery does not work. Messages like the following are logged into '/usr/local/psa/var/log/maillog':

postfix/smtpd[22697]: warning: connect to Milter service inet:localhost:12768: Connection refused
postfix/smtpd[22697]: NOQUEUE: milter-reject: CONNECT from unknown[]: 451 4.7.1 Service unavailable - try again later; proto=SMTP

Premium Outgoing Antispam is installed on the server, or was installed and later removed.


Premium Outgoing Antispam uses a special milter service that should listen on port 12768. Additionally, the issue may be caused by one of the following mail handlers, which is working through the postfix milter:

Kaspersky Antivirus
Dr. Web
dk_check (domain keys)

There are two possible causes of the issue with Outgoing Antispam:

  1. The CTmilter service has crashed
  2. Premium Outgoing Antispam has been disabled, but milter is still present in the Postfix configuration:

    # cat /etc/postfix/ | grep -i milters
    smtpd_milters = inet: unix:/ctmilter/ctmilter.sock
    non_smtpd_milters = unix:/ctmilter/ctmilter.sock


Make sure that you have Premium Outgoing Antispam installed on a server as this article applies to only servers with installed Premium Outgoing Antispam. Go to Extensions to check if you have it installed:


If it is not installed, visit:

#114982 "451 4.7.1 Service Unavailable - Try Again Later" Messages in the Maillog

Below instructions are only for servers with Premium Outgoing Antispam installed:

  1. Start the outgoing antispam milter service or reinstall it if required:

    # /etc/init.d/ctasd_initd start
    # /etc/init.d/ctmilter_initd start

    Check the following article, which may include information related to the failure: #117726


    Check whether the psa-pc-remote service is started.

    # service pc-remote status
  2. Default milter configuration without Premium Outgoing Antispam should be the following:

    • on Debian-based OS:

      # cat /etc/postfix/ | grep -i milters
      smtpd_milters = inet:
      non_smtpd_milters =
    • on RedHat-based OS:

      # cat /etc/postfix/ | grep -i milters
      smtpd_milters = , inet:

Search Words

550 5.7.1 Command rejected

NOQUEUE: milter-reject:

451 4.7.1 Service unavailable

too many open files

SMTP Error (451): Failed to set sender


greylisting slow


postfix/smtpd: NOQUEUE: milter-reject: Service unavailable - try again later; proto=SMTP

milter-reject service unavailable


not sending email

Premium Outgoing Antispam


checking logs for incoming emails

smtp delay

outlook has waited and the server did not respond

smtp not working

451 4.7.1 Service unavailable - try again later;

authentication error: Input/output error

SMTP Error (454): Authentication failed


Postfix mail stuck

Error sending/receiving email

postfix is running on the server but it is not running on the plesk control panel

SMFIC_BODYEOB reply packet header

connect to Milter service inet:localhost:12768: Connection refused

Service unavailable - try again later

Problem with mail

Postfix performs service stops during the day, we can not diagnose the problem log.

spam gmail

mail delivery fails

END-OF-MESSAGE service unavailable

451 4.7.1 Service unavailable - try again later

postfix slow deliver

451 4.7.1 Service unavailable - try again later milter-reject

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