Here's what you do...
1. Go to the Mail menu, then Preferences. Click on the Accounts tab. Select your account and click the delete account button(a minus sign) at the bottom left hand corner of the Preferences window.
2. Click the add account button to the left of the delete button to set your account back up.
3. When prompted, choose .Mac as account type and fill in the rest of the info that is required.
4. When completed, click the red close window button to save your changes and close the Preferences window, returning you to your Mail viewer window. It helps to quit Mail and relaunch it. That's it. .Mac accounts are actually the easiest to set up, so you should be all set after you complete those steps. Let me know.