I also have a custom domain on fastmail that seems to end up in the spam folder of people using outlook in an enterprise setting. Kind of unsettling. I have never had it happen in any other case that I know of. But I’m pretty sure I have gotten spam flagged even when they have sent an email to me first. And I also think I have been OK’d at some point by them flagging me as not spam.. I talked to fastmail support but all they could do was verify that my domain settings were indeed correct