99% of the time it has to do with the actual domain names\ reputation. There are many ways to improve it but we are not experts so we leave it up to those that are.
Two articles we have found will help you fix deliverability issues:
Good Practices for Email Deliverability
https://www.sparkpost.com/features/email-deliverability/
https://sendgrid.com/resource/email-deliverability/