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.
And as reported in chat transcripts, it appears that @mac.com addresses are effectively "abandoned" by Apple and will soon be entirely replaced by @me.com addresses. Unsurprising, but the whole transitioning process requires a quite complicated and convoluted manual setup. Basically, firstname.lastname@example.org email addresses will become aliases to email@example.com accounts, which need to be set up manually. However, according to the discussion thread, sending emails from @mac.com addresses will no longer be possible in the future both from MobileMe's website and desktop email programs:
Customer 5:37 p.m.:
I understand that. But what about sending email from firstname.lastname@example.org? Will that no longer be possible from www.me.com and my desktop email program in the future, like it is now not possible from my iPhone?
Advisor 5:38 p.m.:
Basically that will be like that.
Customer 5:39 p.m.:
So, to confirm: Apple will be turning off the ability to send email as email@example.com from the me.com website, and desktop email applications?
Advisor 5:40 p.m.:
Users who were subscribed to the .Mac service before the MobileMe change in 2008 should still have both @mac.com and @me.com addresses, thanks to the transitioning system put in place by Apple years ago. Expired accounts and new accounts, as you know, can't have @mac.com email addresses. Issues with old @mac.com email addresses on the most recent version of iOS have been reported by other users on Apple Discussions as well.