Unable to change iCloud email settings in Apple Mail on MacBook Pro

I have setup a new MacBook Pro with an Apple ID that ends with a non Apple domain. Apple Mail has decided that this is the default iCloud address and won't let me change the address, delete the iCloud account setup in Mail or manually add the non-Apple email address as a new account.

What I want to do is:

1) Change the iCloud account settings in Apple Mail to an existing xxxx@icloud.com address or, alternatively, delete the iCloud account in mail and recreate it from scratch.

2) Manually set up the email account that is tied to the Apple ID.

Any ideas?