I've been trying to solve the problem with the switch to the greyed-out "p99-imap.mail.me.com"--presumably somehow made overnight at Apple's server end of things--and the concomitant lack of iCloud e-mail "push" updates on three Apple devices (iMac, PowerBook Pro, and [original] iPad) since around 10 AM CDT.
After trying everything I could think of as mentioned in this thread, I just now tried entering both "mail.me.com" AND "imap.mail.me.com" in the Mail Preferences > Accounts > Advanced > IMAP Path Prefix box. I made sure to save this change each successive time when instructed to as I clicked out of the Preferences window. These manipulations took place around 3:15 PM CDT.
Nevertheless, the Incoming Mail Server box under Preferences > Accounts > Account Information still remains greyed-out, and still reads "p99-imap.mail.me.com. And I still have no Apple Mail service on my iMac, my main computer, although throughout the day I've been able to get my e-mail by logging into iCloud on the Apple website.
It looks to me as if those who've said that this has to be a problem with Apple's mail servers are correct. It does boggle one's mind, though, that Apple technical support can't fix this problem in a timely fashion. My patience is wearing thin! E-mail on my iPad was already acting oddly when I checked at 11 PM CDT Thursday, March 22, by the way.