Cronjob + excess CPU make for no mail delivery

Which version of phpList are you running? 3.3.1 / amazonAWS on port 25/ Siteground hosting / PHP version 5.6
Which version of which  browser are you using to view your site? Latest Firefox browser
A link to your site. https://ifeaistanbul.net/

Hi everyone,
We upgraded last week to phplist 3.3.1. We have been using phplist since 2013 without problems once we figured out how to properly configure it.
We have received precious help from this community, and our host has also been very helpful.
For a couple of months, though, we have been plagued with excess CPU usage. Host finally said the cronjobs are responsible of this high consumption.

What we see on our end is the mailing job being sent over and over again. We are sending every month a mailing to 4,000 and another to 3,5000 subscribers, but the task is processed as follows
2 months ago

  • Time to send: 2 days 9 hours 30 minutes 05 seconds
  • Total 20553
    or, for the last one I canceled before "completion"
    Current
  • Time to send: unknown
  • Total 209928

The log of events tells me this current batch has not been delivered at all, which explains why the task keeps running

Error sending email to mail@mail.com SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting

My host has since cancelled the cronjob responsible for the high CPU consumption and asks for me to verify the way the rule has been written

CMD: mysite /usr/local/bin/php -q /home/mysite/public_html/admin/index.php -pprocessqueue -c/home/mysite/public_html/config/config.php >/dev/null

I do not know whether the problem lies 100% with the cron, as it has worked perfectly for nearly 4 years.
The upgrade to 3.3.1 seems to have made things worse: some months ago, the batch would go on a few iterations but would eventually be sent, while now, we send 0 emails but the job is on an endless loop.

I have tried to review the config file regarding SMTP, but found the recommended parameters are already in use
Our batch processing rate is 1000 msg/hr, about which our host has never complained so far
I also tried to look for similar issues discussed on the forum, and found this thread but could not find an explanation or a fix.

Do you have any advice on this matter?

This seems to indicate that you have something wrong with your mail server details in the phplist config file.