Can't set up iCloud email in Apple Mail

On my Mac, I turned on email in my iCloud settings [System Preferences > iCloud]. I went through the procedure of setting up an email address etc. When that was finished there was a new account in Apple Mail. But that account has a number of problems:

• It's a POP account instead of IMAP.
• Incoming Mail Server is my ISP instead of iCloud.
• Is offline and I can't get it to go online - I get this error in Connection Doctor "Trying to log in to this POP account failed. Verify that the username and password are correct.

I can't find any way to change the user name. Also, I can't find a way to change the password.

Apple's support pages say that iCloud email can't use POP ! So, why is it set up as a POP account ?

Weirdly, outgoing email works. I can send messages from the iCloud account in Apple Mail but I can't send (because it's offline ?).

I've tried turning Mail off and on again in iCloud settings but nothing changes. I can't figure out how to delete the entire email account from iCloud so I can start again.

How can I set up iCloud email in Apple Mail to be an IMAP account and with the correct incoming server etc ?