How can I get IMAP email clients e.g. Mac Mail configured so that can receive and sent emails on iCloud+ custom email domain accounts?

I was delighted to hear that iCloud+ subscriber, I get up to five custom domains with three email boxes per domain. I have configured a domain and with several email addresses and successfully updated the DNS records to point at the iCloud servers. However, the mails to my new add on custom domain are showing up only in my primary Apple ID domain email box in Mail. This not much use to me. I thought there were independent email IIMAP accounts email boxes that I could attach to and, possibly with another email client through by typical IMAP/SMTP configuration. However, this does not seem to be the case.

When I have tried to configure in either the email client Mac Mail or Spark using an iCloud connection is always default to be primary Apple ID domain If I try to an a manual set up using imap.mail.me.com and smtp.mail.me.com it fails. Probably because I don’t have a password for the IMAP server? I have tried my primary AppleID password and in the case of Spark the Application specific iCloud password but nothing works.

Is this really a separate email inbox domain, or is it something that I'm going to have to just manage through Smart inboxes in which case it's useless to me because I cannot send using that email address. It states clearly that these are not aliases, but independent email boxes. And so therefore, it should be possible to independently send and receive from those email boxes. How do I do that? Please? The attached images show 1) my custom email domains set-up in iCloud and 2) Failure to connect using Mac Mail manual add account. 3) Failure to Connect using Spark.

Note help page on spark says: "**Note: ** App-specific passwords work only for the iCloud email address with the @icloud.com, @me.com, or @mac.com domain name. This address can differ from your Apple ID email address (e.g. your Apple ID may be linked to Gmail, Outlook, or other emails). You can see the iCloud email address you need in the “Reachable At”/“Contactable At” section section on the Apple ID webpage." But I have no idea what this means.

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