Using classic Outlook for Windows in business environments
Hello! Thank you for contacting Microsoft. My name is Ben and I am an Independent Advisor and as well as a long-time Microsoft user.
I hope you are doing well. With regards to your issue with the iCloud custom domain, it may need necesarry settings to be setup on the Outlook windows mail app, here is what I found but based on the Support page of Apple which is your provider https://support.apple.com/en-gb/guide/icloud-wi...
The link includes guide to set it up on Windows device. I hope it helps but do let us know if you have questions.
Kind regards,
Ben