I can help you to configure your system the way you want.
The issue you are experiencing is common. Big email providers like Gmail, Microsoft have ramped up their fight against spam, consequently, they have been getting stricter in filtering emails that looks fishy.
Your email would have pass through with the exact setup (same infrastructure) a month ago, while that email would bounce now. It's not that you have introduced a new setting in your setup but it's the email providers getting smarter and working together to apply stricter policy. It's good indeed.
It's good that you already have your dedicated IP, but it's not important. What is important is your domain.
You will need a domain that you have control of / that you send emails on behalf of.
I will guide you what needs to be configured for your email to go through.
I manage about 50 learning management systems for customers. I have been helping them on sorting out email delivery issue. So, I have a proven process for you :-)
Cheers
Sijan