Confused about my @mac.com email account and iCloud, etc.

I “verified” my old Apple ID, ******@mac.com, and according to iCloud: About your @icloud.com, @me.com, and @mac.com email addresses, I ought to be able to send and receive email via Mail 8.2 on my MacBook (Yosemite) and access that email via iCloud as well. I logged into iCloud using this @mac.com address and was immediately greeted by this message:

Mail _is_ turned on in the iCloud Preferences for my primary Apple ID. There’s no way to create a new iCloud email account there. So, I created the new @mac.com account in “Internet Accounts,” and immediately got the following:

followed instantly by this:

What’s going on here? If I create an @iCloud.com address with the same user name as the @mac.com address, will the iCloud account replace the mac.com account? Will I get my @mac.com email from this new iCloud account?