how do i remove old email as apple id

Good Evening, I'm trying to get rid of a troublesome email address which happens to me apple id, I've made an iCloud email address to and I've added this to my account as it now stands as an alias in my account.

I would like to remove the old email address now but it won't let me as it keeps saying I can only use an iCloud.com email address that is added to my account?

I have an iPhone 7, iMac and Mac book pro and they all seem to list slightly different reasons why i can't delete this email as my apple id? I can now login with my new iCloud email address but it won't let remove the old one?

Can someone please advice, it would be a great help

Many Thanks Daryll