MAIL will not allow iCloud alias to receive mail

Question

I am not a novice. I have been on iCloud (sadly) since July 2011. Today I tried to activate an email alias which was transferred into iCloud. It's valid and has been for years, but not activate since I migrated to iCloud.
Every time I set up the existing alias in MAIL I get the error "invalid incoming mail server."  I have used the Apple recomendation to use imap.mail.me.com.  When it comes time to exist ACCOUNT under Mail PREFERENCES, it will never SAVE changes as the incomding mail server information is grayed out and goes back to "example" and is not accessible.
I have also entered p99-imap.mail.me.com with the same result.
Two hours of going to icloud.com or managing my apple id online have brought exactly the same result.  Although the alias shows as activated under icloud.com and although it is accepted by MAIL, the message is always the same (invalid incoming mail server) and the account change can never be saved in MAIL.
XXX
I have a business with MobileMe accounts and we are going to be forced to migrate to icloud in a few months. I have nightmares about it alraedy.
Since Apple doesn't give a good cr*p about this, I humbly ask if there is anyone who really knows what is going wrong and how to work around it?
Again: I migrated to icloud with my apple id in July 2011.  MAIL will NOT set up my icloud alias as it always grays out the incoming mail server and returns the error "invalid incoming mail server" and will not even allow me the save the information.
Please help. 
Thank you.
Mke

Answer 1

Hi Roger
Thank you for your suggestions.
I already have an ALIAS set up and receiving emails in my main INBOX under the alias name. It is a MobileMe alias from my separate business mac account.  It has worked as a separate "account" under MAIL for 10 years. It is an ALIAS, not a main identity mac account. The System Preference panel is ACCOUNTS, not aliases, so I use the word "account" when I speak of setting up a new inbox with a different name and password whether that is an iCloud alias or a gmail account.
We may be talking about the same thing but I'm not using a word you use.  I am allowed 3 icloud mail aliases. I am allowed 5 mail aliases under my business mac account. On my mac MAIL I already have three separate "accounts" or inboxes:  (1) my private mac ID account  (2) my business MAC account  (3) an alias from my business account. All three have different passwords. None of the three gets emails sent the other two "accounts."
Maybe I just don't get "icloud." I certainly got "MobileMe" in the past. The alias that is currently working as a separate "account" or "identity" under MAIL is proof of that. 
I am going to delete the "alias account" from MAIL and call it a day. For me, iCloud is commensurate with Windows ME in the history of computing: a stupid change that resulted in nothing but aggravation for everybody who was dumb enough to sign up for it.  I won't be signing up for the next "big thing" from Apple. I'm sure there are tens of thousands of other users who feel equally "used" by Apple on this.
I'm marking the question ANSWERED and closing the thread.  iCloud is iCrap and nothing I've experienced with it in six months has changed that opion and the fact that you can get a gmail account operating in MAIL in 2 minutes but you can't get an iCloud email alias working in five hours is just more fuel on that bonfire.
Thank you again for your couresty and your time.  I doubt if my business account, which has to be downgraded to iCloud in a few months, will function at first and I doubt if the 10-year-old alias on that account will continue to function in MAIL. 
Mike