Can't send emails from Custom Domain on third party clients

Hello,

When using third party apps on Windows, I can't send emails from my Custom Domain (i.e. @xyz.com) which I host on my iCloud+

this works find when sending emails from iCloud.com or Apple Devices but when I use Outlook on Windows, I can not choose my custom domain as the sender email when composing new emails or replying to emails sent to my custom domain (so i actually receive emails sent to it, i just can't replay using the same address), instead i have only one option and it is the xxx@icloud.com

here is the snippet from Apple website that mentions it should actually work

Because your custom email domain uses iCloud Mail, it works in the same way. When you configure or create a new address, you can automatically send and receive email from that address in Mail anywhere you have iCloud Mail turned on, including your iPhone, iPad, iPod touch, and Mac. You can also access your mail in Outlook on a Windows computer and in a web browser at iCloud.com.

• I am using Outlook free version, not the paid one coming with Office365, right now it is called Outlook new, maybe that helps detect the issue?
• I tried adding my custom domain email instead of my @icloud.com account using iCloud IMAP settings too, but it didn't work

also please don't advice me to contact Microsoft as this is seems to be 100% Apple Problem :(