If you send a mailing test at mail-tester.com you'll see various alerts:
  1. Your message should contain an <html> tag
  2. You should also include a text version of your message
  3. Your message does not contain a List-Unsubscribe header
Those goes hand in hand with the need of an unsubscribe link which is still planned since a year: https://bmrng.canny.io/roadmap/p/need-of-unsubscribe-link-option-to-be-compliant-with-canspam-act-of-us
Without those enhancements in place we are simply burning money when sending mails because no matter what you end up in the spam folder.