I still have it set up with the domain I tested in this article and it works fine if you like iCloud Mail.
Wow, I’m a bit slow to reply! Sorry I didn’t see this until now :) Anyway, I’m looking to transfer from my current reseller web hosting (a service I no longer provide) to a provider who just offers an e-mail only option with custom domains. Given I’m paying for iCloud+ anyway I think that Custom Email Domains with iCloud could do the trick.
No, I haven’t got around to doing this yet… good things take time and inertia is a thing!