[tatersolid]

That might be from a Microsoft feature to prevent phishing that blocks display name spoofing. I get hit with that when I email from my personal to work email… the display name portion of the FROM address matches my work so it trips this filter.

Obviously they can only do this for unique-enough names and so this filtering could never work for “Joe Miller” but it does stop the dozens of phishes we see per day that are FROM our CEO’s first and last name but with a Gmail email address.

[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.