Why won't Mail.app connect to iCloud mail?

I'm having a problem getting Mail app to connect to iCloud, iOS and iCloud web mail seem to be working fine with my @me.com/@iCloud.com address but Mail app on my mac is showing wh I try to add my account:

The iCloud IMAP server “p03-imap.mail.me.com” is not responding. Try checking the network connection, and that the server name is correct. Otherwise, the server might be temporarily unavailable. If you continue, you might not be able to receive messages.

Gmail has/is connected fine, why not iCloud?

Thanks!