Thank you for the reply.
Our phplist application is on a web server connecting to the mail server on port 587 using TLS in order to submit mails. If I understand your comments right, then the phplist application on the web server tries too often to connect and these connections fail.
My phplist settings currently are:
Will you please advise me whether there are any conflicts or other issues with the above?
What should I try to change, based on your experience?
Thanks in advance,