Your unknown domain is suspicious because it as a low reputation means « We don’t like you because we don’t know you so we consider your mails as spam ». There is another word for that; xenophobia. Whatever…
If you use another « beautiful and know other big company to relay your mails » you will be welcome and your mail would go to Inboxes instead of spam folders because those other big smtp server are known and welcome.
And their business is also related to marketing so… you know… profiling and making money with it it’s a doublé. You have to use them to deliver a mail because they don’t know your domain and they make money by tracking people and selling data about it.
Just do it… and you will be relieved… but for me it’s like taking the Blue Pill and staying in the Matrix.