After you read a message, you can delete it if you don't need it any more. Here's how to delete a message:
Start the Mail program. The Mail window appears.
Click the Inbox folder under the Mailboxes category in the left pane.
Click a message you want to delete. (To select multiple messages, hold down the key as you click each message that you want to delete.)
Click the Delete button (or select Edit ▸ Delete).