With a simplenews / symfonymailer integration I send 20 newsletter mails per cron run. Everything worked so far until I run into a "Attempting to re-run cron while it is already running". I checked for solutions and then used the cron in drush with with drush core:cron -l mywebsite.
Result was that all images were missing. All urls became the mywebsite/node/xyz instead of having a full URL like https://www.mywebsite.com/node/xyz.
So Drush can not be used to run the cron for sending mails with simplenews.
Not sure if this a simplenews problem or symfony mailer?
Active
3.0
Usability
We also had a problem with "Attempting to re-run cron while it is already running" and broken URLs.
We use drush cron and -l / --uri param which works fine normally, but cron occasionally gets triggered simultaneously from external visitors. These requests came through a reverse proxy which maybe triggered the broken host names being internal names for the newsletters.
Maybe this info helps you in finding the source of the problem.