Article ID: 121000, created on Apr 10, 2014, last review on Jun 9, 2016

  • Applies to:
  • Plesk for Linux/Unix


  1. Outlook runs fine most of the day then out of no where, I get this message:

    Task ' - Sending' reported error (0x800CCC0f): 
    'The connection to the server was interrupted. If this problem continues, contact your server administrator or Internet service provider (ISP).'
  2. Symptoms of Postfix overloading:

    2.1. An increased number of "lost connection after CONNECT" events in /var/log/maillog. This happens because remote SMTP clients disconnect before Postfix answers the connection request.

    tail -fn0 /usr/local/psa/var/log/maillog | grep 'lost connection'
      9 19:15:39 ns14 postfix/error[3815]: 3C65434B1B3: to=<>, relay=none, delay=1.7, delays=1.7/0/0/0, dsn=4.4.2, status=deferred (delivery temporarily suspended: LOST CONNECTION with[] while sending RCPT TO)

    NOTE: port scanning for opened SMTP ports can also result in "lost connection" logfile messages.

    2.2. Warning events are in /var/log/maillog:

    # grep 'has reached its process limit' /usr/local/psa/var/log/maillog
    Apr  9 17:15:56 ns14 postfix/master[2455]: warning: service "smtp" (25) has reached its process limit "100": new clients may experience noticeable delays


Problem on client's side (incorrect settings, networking issue, antivirus or firwall blocking) or mail server overloading.


  1. Sometimes, when e-mail messages are sent via Internet service provider's (ISP) SMTP server, the connection may be timed out. Use article to fix it.

  2. Check all possible causes -network, antivirus, firewall issues - described in the Microsoft article Make sure that email account settings in Outlook are correct.

  3. Under normal conditions Postfixresponds immediately, when an SMTP client connects to it; amount of time to deliver mail is noticeable only with large messages. Performance degrades dramatically when the number of SMTP clients exceeds the number of Postfix processes. When an SMTP client connects while all Postfix processes are busy, the client must wait until a server process becomes available.

    3.1. In case the Postfix server has overload symptoms, the automatic stress-adaptive behavior can be activated.

    3.2. To avoid the "all server processes busy" condition, serve more SMTP clients simultaneously. To do it, increase the number of Postfixprocesses. This will improve the responsiveness for remote SMTP clients, as long as the server machine has enough hardware and software resources to run the additional processes, and as long as the file system can keep up with the additional load. To check the current number of sptpd processes:

    # ps auxwww | grep smtpd | wc -l

    The default_process_limit configuration parameter gives direct control over how many daemon processes Postfix will run. As of Postfix 2.0 the default limit is 100 SMTP client processes, 100 SMTP server processes, and so on. This may overwhelm systems with little memory, as well as networks with low bandwidth.

    You can change the global process limit by command:

     postconf -e default_process_limit=300

    3.3. Review all others solutions described like disconnecting suspicious SMTP clients and improving Postfix server responsiveness by eliminating delays in the article

The following link can be also helpful: Postfix Performance Tuning

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