You’ve upgraded your Mac family to iOS 5 and suddenly your MobileMe/iCloud email is not functioning. A quick search of the Apple Support Communities shows that this is a common issue. In fact, it is so common that there are multiple threads of anger regarding this problem. however, what is lacking is a solution…
Here’s the Fix
(1) Delete the iCloud account. That’s right; get rid of it by going to Settings >> iCloud. Scroll to the bottom of the screen and press “DELETE ACCOUNT.” Don’t worry because the beauty of this new upgrade is that it makes syncing your phone so easily. you may choose to keep all data.
(2) Restart your phone. Hold the power button down until you are given the opportunity to turn it off by sliding the power button on the screen. After it shuts down, restart your phone.
(3) Re-add your iCloud account. Navigate back to Settings >> iCloud and log in . At this point everything should sort itself out and you should be all set. If not, try it again. It worked for me on the first time on my phone but on my iPod Touch, I had to do it twice.