A work around!
Not taking credit, found in different thread, but it saved me (allowed me to get a important email I was waiting for)
In the desktop Mail client, I setup a 2nd account using my iCloud username/password.
It gave me the error everyone is getting, but then went to the next screen. In email type, INSTEAD of picking iCould or MobileMe, I picked the generic IMAP email type
Put in the following....
Incoming
imap.mail.me.com
iCloud username (in my case my @mac.com email)
iCloud Password
Outgoing
smtp.mail.me.com
iCloud username (in my case my @mac.com email)
iCloud Password
This worked for me, download all my email and allowed me to at least see what was in my inbox