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Interesting well it has happened some times even with me as well and we didn't go further and restricted it to 600 because it's a waste of time on our efforts since we run marketing automation.
let me know if you need some help with marketing automation platform
did you try your hands on mailget, aweber, moosend, campaign monitor. I think sharpspring mail+ or mailerlite would help you since you want to send such a large bulk of emails.
Make sure your spf and dkim is setup - in short sender policy framework which is being managed by sender score an IP reputation tool - would help to avoid getting your emails in spam