I don't think it is a problem with independent or self hosted email. I have this problem sending through: * An email service such as Mailchimp * My own serve

I don't think it is a problem with independent or self hosted email. I have this problem sending through: * An email service such as Mailchimp * My own server with SPF / DKIM and good reputation / deliverability for other domains * Amazon AWS Simple Email Service I think it is something on the domain level – perhaps they've flagged the domain.