Might've had something to do with the state of the various email security measures on the domain. I have an Apple account on a custom domain with Fastmail and it's never been a problem.
Mine was also managed by Fastmail. And no one else has ever had a problem with it (including Apple when I added it after signing up).
Mine was also managed by Fastmail. And no one else has ever had a problem with it (including Apple when I added it after signing up).