> Oh, there's a way out of all this though: obtain a "Return Path Certification" [1]. That is, pay them an absurd amount of money and your mails are guaranteed to get to the users' inbox unless you are clearly spamming (all of the above assumed you are NOT).
Bonus: They straight up declined my business because it was too marketing-y for them.
What was too marketing for them? A 100% opt-in list that reminded customers when a product was in stock. Fully made in-house, with captcha, privacy policies, opt-out, and with the user typing their email and name, the complete works for a one-time reminder email.
Could you name the cost?