I’ve been thinking of moving my main email from Fastmail to iCloud for ages. It seems a bit silly to be paying for two email services, although Fastmail offers a lot more power-user type features — none of which I really use.
Apple’s instructions are fairly straightforward, but I ran into a problem when adding the MX, CNAME and other records to my domain, which is registered with A2 Hosting. It seems the DNS Management section on the main client login page has little to do with managing DNS. You have to login to cPanel and create the entries in the Zone Editor. The only other trick is to remove the quotes from the SPF entry that Apple supplies.
Although this in not hard hard to do, it’s still a lot of hoops to jump through. It seems there’s been basically no progression in making creating and using custom domains easier to use in the last two decades.