Configuring preferences and options in Outlook.com
Hi,
Does https://account.live.com/names/manage page not list the custom domain?
Also check:
https://outlook.live.com/mail/options/premium/features
If you find the domain and are unable to remove it, please let me know and attach a screenshot to help me understand the problem better.
I look forward to hearing from you to get the results of the troubleshooting.