Do I have a support account ready on your site? Will you link me to wp-admin privately so I can check out your work?
One cron every 15 minutes will have a large impact on sends to large lists. We can only process ~30 emails at a time, because each has to be generated. Our heartbeat is designed to run in the background every 2 minutes. ~30 emails processed every 15 minutes for 1000 would take around 8 hours to send them all I think.
If you can, you’ll need to bump the natural cron ping cycle back up to at least 1 per minute.
I have tried to send 2 test emails to myself (work and personal email). I have not received any emails though. Looks like all cron tasks are being completed. Inside sparkpost it looks like emails are being sent but only 1 has been accepted. And in the Inboundnow Sparkpost settings it says DKIM sending domain is invalid. Could this be the reason why?
I created a test blog post and it seems to have worked. I received the notification email. Will post a brand new blog post tomorrow and have created a rule that will send to the entire list, so I will see then if it goes through without any issues.