Hey @s2imon. Lets narrow this error down to a specific place in code and see what kind of data is causing this error in your specific case.
From the logs you provided it all comes down to this line throwing the error - https://github.com/TryGhost/Ghost/blob/3.9.0/core/server/services/bulk-email/index.js#L99 .
Lets can put additional logging in your instance’s code to figure out what Ghost is trying to send out. Add following code on line 97:
common.logging.info('Recipients count: ', toAddresses.length);
common.logging.info('Recipient variables: ', Object.keys(recipientVariables).length);
This would give us a clue on the amount of emails that are going out. It’s best if you could run this code on your local or experimental instance with the same error to avoid problems on production.
You can also try and log other variables that are passed to
mailgunInstance.messages().send() function to see if there’s anything out of normal with the post you are trying to send emails to.
The best way to tackle this problem is having a reproducible steps on a clean Ghost instance which you can set up using
ghost install local on your computer. Otherwise it’s taking too much time to guess what is going on here