If you've got an automatic spell-checking function when you post on here, it's generated by your web browser. Similarly, if you access your email through your browser (rather than through a dedicated email client, such as Outlook Express, Windows Live Mail or Thunderbird) it will again be your browser that provides the spell-checker.
So the obvious question has to be 'Which browser are you using?'
(Once we know that, we'll hopefully be able to tell you how to enable spell-checking within it again).