My first try configuring e-mail with Flarum I tried mailjet, because I already have an account there. They're pre-blocking flarum's registration e-mail because it thinks the receiver will block as spam.
My workaround was using my gmail account smtp, but that wouldn't scale in a large forum.
I don't have any large forum to run anyway, and there is a good chance I misconfigured it, but what are you using for sending e-mails? You run your own mail server?