Hi Molly,
I'm thinking they might slowly be warning people that mobileme is going away , & you have to switch by June or so. And, it seems many iDevices have this detection or change worked in, but not Macs running older OSX versions.
Might as well setup a new account now.
iCloud Mail setup, do not choose .mac or MobileMe as type, but choose IMAP...
IMAP (Incoming Mail Server) information:
▪Server name: imap.mail.me.com
▪SSL Required: Yes
▪Port: 993
▪Username: example@me.com (use your @me.com address from your iCloud account)
▪Password: Your iCloud password
SMTP (outgoing mail server) information:
▪Server name: smtp.mail.me.com
▪SSL Required: Yes
▪Port: 587
▪SMTP Authentication Required: Yes
▪Username: example@me.com (use your @me.com address from your iCloud account)
▪Password: Your iCloud password