I can get it to work with my Mac client (Monterey beta) but it will not work for me on iPhone or beta.icloud.com .... just checked my iPad and it does not work there either.I have set it up however it doesn't seem to work on Mac Mail Client? Anyone experiencing the same issue? I don't get to see my custom domain in the Mac Mail Client.
it is beta, I suppose.....but Apple generally are a little further along when they test things like this.