[wakawaka28]

I don't think it's anti-phishing. If it is, then it's a broken rule. I have registered FastMail as my mail server properly with my DNS (MX record, I think) and also done the other authentication stuff. This is not a case of merely setting the FROM field, as many people do. You can set the FROM field of a message to anything if your mail server allows this. The receiver might reject this, but it is/was a common way to configure your mail client in the past because of stuff like email forwarding. Lots of people use 3rd party mail providers whose servers are not under the same domain as the mail being served. I think most people with custom domains do not run their own mail servers, and thus point their MX entries at a 3rd party mail provider just like I do.

I don't know why this stuff would be rejected. I went through several debugging steps online and didn't get anywhere with it. Every tool said I had set it up correctly.