I can’t tell you how long I have been waiting for this. Finally Apple are allowing custom domain names to be used on iCloud. See https://www.macrumors.com/2021/08/25/icloud-custom-email-domain-feature-in-beta/
I’ve always been confused while Apple hasn’t offered this service sooner. There have been workarounds, forwarding your custom domain emails to your iCloud account etc. But I am not a fan of workarounds.
The main things I like about Apples email service is that the emails are PUSH rather than FETCH, which means emails arrive in my inbox instantly and the mail application uses less battery life.
Using Apples BETA site https://beta.icloud.com/ I added a test domain to my account and within a few minutes could send and receive emails via my domain, fantastic.
I noticed a few small issues, which can be forgiven in BETA. The instructions for adding MX records to your DNS weren’t as straight forward as they need to be, for example there is no mention to which priority you should set for the two MX records.
Secondly, when I sent a few emails back and forth to my other email accounts, some of the replies to from my iCloud account were subsequently missing from the conversation view, even though visible in the SENT folder.
And my only gripe is there is a character limit to alias names on the domains. I like to use firstname.lastname@example.org but when I tried to set this up I received a warning to say the me@ alias was too short and I should try another. As I would eventually want to move all my email to iCloud this would be a massive issue for me as me@ is the primary email address used on all my domains. Lets hope Apple see the issue and remove this unnecessary restriction.
Looking forward to September when they role this out in their IOS 15.0 update.