I have a similar problem. I used to own a .mac email address, which I had moved to MobileMe for a while then discontinued. However, I made an Apple ID when I had the .mac email address.
So, I had
xxx@mac.com as my Apple ID.
xxx@me.com was my old MobileMe ID, which I discontinued.
When I made iCloud ID, it defaulted to:
xxx@mac.com and it made email address at xxx@mac.com and xxx@me.com
Then I realized I couldn't send email from Mac using xxx@mac.com at all. I was able to send email with xxx@me.com ID. No big deal.
Problem is that unlike OSX Lion email, I cannot set up iOS 5 email to ignore xxx@mac.com and only use xxx@me.com! Email is stuck at xxx@mac.com and I am not allowed to send any emails. I can receive emails addressed without any problems, but I cannot send any emails.