According to a recent thread on Apple’s Discussion boards and a support document on Apple’s website, users of @mac.com email addresses who upgraded to iOS 4.2 are no longer able to send email messages through a @mac.com account unless it was setup prior to updating to iOS 4.2. Alternatively, the @mac.com account details can be synced through iTunes on the desktop, but this won’t enable push for emails.
A user on Apple Discussions explains:
Because of this unannounced development, I had a sneaking suspicion that Apple may be planning to do the same in the future for sending email from @mac.com addresses via www.me.com and a desktop email client.
It appears my suspicion was true. Here are the relevant sections from a Chat Session I’ve just finished with a very nice MobileMe Support agent.