You can delete an e-mail account when the account is no longer needed. You can not delete your default account.
Warning: Make sure you download any mail that you want to keep, as deleting the account will delete all of the mail still in that account. You can not recover this information once the account has been deleted.
To delete an e-mail account:
Click on the Add/Remove Accounts link in the Mail area.
Click on the Delete button next to the unwanted e-mail address.
Click on the Yes button.