SMTP Error: The following recipients failed: 12345@*.com:

From my Log of events:

143192 2016-10-09 09:00:03 Finished this run
143191 2016-10-09 09:00:03 305 failed (will retry later)
143190 2016-10-09 09:00:03 1 messages sent in 3601.95 seconds (0 msgs/hr)
143189 2016-10-09 09:00:03 Error: Process Killed by other process
143188 2016-10-09 09:00:03 Error sending email to 12345@.com SMTP Error: The following recipients failed: 12345@.com:
143187 2016-10-09 09:00:01 Looking for subscribers
143186 2016-10-09 09:00:01 Processing message 1
143185 2016-10-09 09:00:01 One campaign to process.
143184 2016-10-09 09:00:01 Processing has started,
143183 2016-10-09 08:59:53 Error sending email to 1235*@.com SMTP Error: The following recipients failed: 1235@.com:
143182 2016-10-09 08:59:43 Error sending email to 1235
@.com.tw SMTP Error: The following recipients failed: 1235@*.com.tw:

From the config file:
define(‘MAILQUEUE_BATCH_SIZE’, 8000); # tested with 2000, 3000, 4000, 8000, 16000 before
define(‘MAILQUEUE_BATCH_PERIOD’, 1800); # tested with 7200, 3600, 1800 before

It started good, … PhpList reported up to 12,000 messages per hour. It got slower and slower. Till it reached about 350 messages per hour. The log file reported 305 failed messages, which have no explanation.
How can I track that down?
Next run retries the same messages and got nothing thru. That repeats all the time.

I collect all these email addresses from the log file and add them to the Suppression List permanently.

The next run starts again with 12,000 messages per hour, … and it repeats as above described.
Since it did not send out the message, there is no bounce!!!

I also found some emails reported as invalid.
I tried to use Reconcile - Delete subscribers who have an invalid email address.
However, this function does not get rid of invalid email addresses.

Sometimes I get an email from my system:
[Sat 8 Oct 2016 17:00] [CL] Started
[Sat 8 Oct 2016 17:00] [CL] Sending in batches of 8000 emails
[Sat 8 Oct 2016 17:00] [CL] Processing has started,
[Sat 8 Oct 2016 17:00] [CL] One campaign to process.
[Sat 8 Oct 2016 17:00] [CL] Processing message 1
[Sat 8 Oct 2016 17:00] [CL] Looking for subscribers
[Sat 8 Oct 2016 17:08] [CL] Found them: 35241 to processProcess Killed
by other process
[Sat 8 Oct 2016 18:00] [CL] Error: Process Killed by other process
[Sat 8 Oct 2016 18:00] [CL] Script stage: 5
[Sat 8 Oct 2016 18:00] [CL] 313 failed (will retry later)

Why only sometimes, shouldn’t I get it with each cron start?
Why is the process killed by other process?

If I try to start the crontab command at the command line, then I get:
phpList version 3.2.5 © 2000-2016 phpList Ltd, http://www.phplist.com
phpList - A process for this page is already running and it was still alive 1 seconds ago
phpList - Running commandline, quitting. We’ll find out what to do in the next run.

I use my ISPs mail server to send out the emails. I do not have any restrictions there, hence I can see the 12,000 messages per hour information.

How can I get my system more stable? My goal would be 8000 messages per hour. I tried several settings, with a cron starting every 5 min.

@Elmit With a cron job running every 5 minutes you should set the batch size to 1/12 of your hourly target, assuming that phplist can send that many within 5 minutes.
Also, set the batch period to a small number, say 5s, so that phplist does not unnecessarily restrict the number of emails that it will send in one batch.