Postfix keeps stopping, preventing the sending of mail: postfix-srs: process id XXXXX: command time limit exceeded

Symptoms

• Plesk Obsidian running on a Linux-based operating system
• Postfix keeps stopping, preventing the sending of mail
• Mail is received with a significant delay
• The /var/log/maillog on the server contains many errors that are similar to the following:

  Feb 24 08:39:58 server postfix/spawn[13353]: warning: /usr/lib64/plesk-9.0/postfix-srs: process id 21094: command time limit exceeded

  2023-03-30 21:32:07 INFO
  postfix/smtpd [3238741]
  lost connection after CONNECT from unknown[203.0.113.2]

  postfix/master[10149]: warning: service "smtp" (25) has reached its process limit "100": new clients may experience noticeable delays
  postfix/master[10149]: warning: to avoid this condition, increase the process count in master.cf or reduce the service time per client
  warning: see http://www.postfix.org/STRESS_README.html for examples of stress-adapting configuration settings
  warning: maildrop/777F954752E9: error writing 8E9F654752EA: queue file write error

Cause

The postfix-srs: process id 21094: command time limit exceeded messages reflect a normal and expected behavior of Postfix, which force closes processes that have not been closed for a very long time.

This usually happens when a mail client does not end its connection to the Postfix as expected (by sending QUIT and "221 2.0.0 Bye) and keeps it open instead.

With that said, this behavior is mostly caused by incorrect behavior on the side of the mail client that is tied to this process and due to this, the mail client's configuration needs to be reviewed and corrected (if possible).

Another possibility for such a situation is an intentional brute-force attack that is meant to cripple your mail server.

When too many such connections are kept open, the Postfix mail server gets overloaded and switches to
stress mode. Postfix version 2.5 introduces automatic stress…