To find a solution, I tested many email service providers, tried to set up a self-hosted email service, and tried various server warming up techniques.
What did I find? You NEVER reach a good inbox rate if you use a POOR-quality email ad copy. I am not referring to spam words, which all major email service providers check for automatically.
For example, you must set up your email header/footer properly, make your newsletter MOBILE responsive, add a limited number of pictures, prepare PLAIN text copy of your email content, and consider other details too numerous to name here.
Your emails should not only look GOOD but should comply with the latest regulations of email marketing.