Oh, thanks for the heads up! I don't send spam - I've just sent 01 email so far. lol
I've been using the Buttondown.email service. But will move to a custom domain to improve deliverability.
I believe they were saying that your site wouldn't let them add their email to the list. Buttondown seems a bit overzealous in not getting bounces in my experience and rejects perfectly valid emails. It's a great service otherwise.