Hi @phill104, AWS throttles port 25 on EC2 and Lightsail instances, Amazon came back to me (when we had to lift the port 25 throttle with the below info:
Remove Throttle on port 25 on EC2 form: https://aws.amazon.com/premiumsupport/knowledge-center/ec2-port-25-throttle/
This will also help with the spam flagging issue, and you can request reverse DNS for your AWS instance IP’s using the same form (https://aws-portal.amazon.com/gp/aws/html-forms-controller/contactus/ec2-email-limit-rdns-request) when you’re logged into your account.
If you don’t have a support plan with Amazon you may be asked to add one, the basic one (around $21/month) is sufficient.
I hope that helps!
– Chris