They could sue but they would probably lose as long as mymacmail.com doesn't create the impression that it's directly associated with Apple. Another example would be a reseller like MacConnection - they can use "Mac" in the name as long as they don't pretend to be anything other than a reseller.
This is partly because, unlike "Xerox" or "Kodak," "Mac" is a word that predated Apple's computers. If Apple had invented the word, it could prevent others from using it.
I could be mistaken - I'm not sure if Apple has ever brought a suit of that nature. It could be that Apple is simply more tolerant than other companies because it wants to spread awareness of the "Mac" brand.
Of course, if anyone else made a computer and called it a "Mac," there's no question that Apple could sue them and win.