I'm trying to add a new alias on my MobileMe account. My older aliases have both @mac and @me addresses but the new one I'm adding doesn't give me an option to use both, only @me.
You can only use a "@mac.com" and "@me.com" alias if you created them before the MobileMe switchover. Therefore, if you created an alias after the MobileMe switchover, you can only use "@me.com".
AIt also says that: "Any aliases you created (and didn't delete) before the .Mac-to-MobileMe should have both @mac.com and @me.com versions. Aliases created after the transition can only be used with @me.com."