Apple is in the process of converting all of its email accounts to @icloud.com. It could well be that this process permanently disconnects older OSX versions such as yours.
Try to log into icloud.com with your old ID using a browser. If you can get in, check the mail page. If you can't then the recent changes have denied you access to the new icloud.
Thanks for the reply. Yes, I can log into iCloud with Apple ID, and I can send and receive mail that way...the problem seems to be with the Mail application, where my outgoing server reads 'offline', no matter what.
Curiously, I got another storage message today saying my iCloud storage is maxed out at 25GB and that I won't be able to send or receive. It's odd, b/c when I check my account status on my iPad, I have TONS of storage left. I don't get it. And I can still send and receive messages via the online service...